When Paula Sherriff stood up in the House of Commons two weeks ago to beg Boris Johnson to moderate his “offensive, dangerous, inflammatory” language, she took herself by surprise. “I know it sounds bizarre, but I didn’t even brush my hair,” she says. She had gone into the chamber intending to listen to the prime minister’s statement – this was the day after the prorogation of parliament was ruled unlawful. But as Johnson spoke, she felt the anxiety intensify in her corner of the Commons.

“I was sitting among female MPs. We were getting irritated. Agitated,” she says. She must feel it now, too, because she has started to fidget in her seat, in her office in Westminster. “We were saying: ‘This is what’s going to be in our inboxes tomorrow!” Every time he said: ‘Surrender’, we were getting more and more … ” she squirms, her arms jostling with the elbows of imaginary neighbours.

So she went to see the Speaker, who told her: “Paula, I’m entirely sympathetic,” but there would be no points of order during Johnson’s statement. “I said: ‘Could I shoehorn it into a question?’, and he said, ‘Absolutely!’ He was unequivocal.”

He called her quickly. “I remember thinking: ‘Oh, shit!’ when I heard my name. I stood up.” That’s when, in a voice breaking with emotion, she told Johnson how the death threats she and her colleagues received often quoted him. “It came from the heart.”

Immediately, the abuse focused on her. Sherriff rattles off a list: “You were shrieking like a fish wife”; “You fat bitch”; “Stupid cow” and “I won’t be happy till you’re hanging from a lamppost.”

Much of it focuses on her weight. She looks down at her iPad and reads an email from a constituent. “I have some advice for Paula Sherriff. If you can’t stand the heat stay out of the kitchen. And if the photo on the front of last week’s [local paper] is anything to go by, the kitchen should be out of bounds for Miss Sherriff.”

Coming from the heart is very much Sherriff’s approach to politics. “I’m a heckler,” she says. Before she became an MP she used to think members were like animals and the House of Commons a zoo. “I always said I wouldn’t do it. But there is something about being in there and getting carried away.” She is reflecting on her own behaviour, but it must seem punishing that her passion is provoking the kind of abuse to which she is so opposed.

Ever since she was a girl, Sherriff, 44, says, she “always believed in speaking out”, but now the urge is taut with the fear that lives are at stake. She became an MP in 2015, the same time as Jo Cox. Her Yorkshire constituency of Dewsbury borders Batley & Spen, where Cox was killed by the far-right extremist Thomas Mair, in June 2016. The library where Cox was attacked is only three or four miles out of the corner of Sherriff’s eye. Do MPs fear another murder? “Yes, I think they do. And I don’t want to look back and think: ‘I could have said something, and I didn’t – and now we’ve lost another colleague.”

Of course, she has had CCTV installed, and alarms. “And you know, in your heart of hearts, that the vast majority of people who send these messages have no intention of following up. They say they want you hanging from a tree,” she says, making an effort at nonchalance, “but they’re not actually going to come and grab you and kidnap you.”

She tries to be rational. But still, “there’s that little seed at the back of your mind”. She mentions the Labour MP Rosie Cooper: in May, a member of the neo-Nazi group National Action was found guilty of plotting to murder her. “Sometimes I come home at midnight. There are times I put the key in the door, walk in, turn the alarm off. You get that little moment. Is someone going to jump out at me?” It used to be that she could leave her house and go to the corner shop, all without looking over her shoulder. But not any more. She looks. “Absolutely, constantly.”

In some ways, Sherriff is still looking over her shoulder at that day in June 2016 when her office manager, Karen Rowling – who is busy at the next desk – phoned to ask if she had heard anything.

Sherriff was enjoying a rare lie-in, having got home from work at 2am. When the phone rang at lunchtime, she assumed that Rowling was pestering her to get a move on.

“I said: ‘I’m coming! I’m just straightening my hair!’” But a friend of Karen’s had been in the pub with colleagues of Cox. One was called away suddenly; there was a rumour that she had been shot. “I said to Karen: ‘Don’t be daft!’ We were quite dismissive, weren’t we?’” Sherriff says, turning to Rowling. “I mean, it’s not London or Manchester or Birmingham. It’s quite sleepy.”

Feeling “a bit stupid” for troubling him, Sherriff phoned the Kirklees chief superintendent. She was sure he wouldn’t answer if something was going on. So when he picked up, she momentarily relaxed. But then he asked her where she was. “I said, ‘I’m at home.’ He said: ‘Can I ask you to stay indoors?’”

They shut the office; sent everyone home. “You came to my house, didn’t you, Karen?” Sherriff says. “You kept shouting at me to get away from the windows. I was just pacing. We waited.”

At 4pm Louise Haigh, the MP for Sheffield, called with the news that Cox had died. “I remember walking into the back bedroom of my house and she was crying. I was crying. We were in shock. I stayed at Karen’s house that night. We shared a bed, didn’t we?” Sherriff says over her shoulder. “I remember waking in the morning and saying: ‘Did it happen?’ It felt surreal. Just devastating.”

Sherriff’s recollections are granular and specific, focused on the small moments – straightening her hair, the back bedroom, the shared bed – as if the brightness setting in her memory of the entire day is switched to maximum. “That day changed everything,” she says.

Paula Sherriff leave St Peters Church in the town of Birstall, West Yorkshire, where a vigil was held for murdered Labour MP Jo Cox. Photograph: Ian Hinchliffe/REX/Shutterstock

After working in prosecutions in the police force for 10 years after she left school, Sherriff thought she “knew a lot about murder”, but the killing of Cox is so far outside her experience, she is unsure if what she feels is “just natural” given the extraordinary circumstances, or beyond the range of normal recovery. “A senior officer did suggest to us a few months ago that we have post-traumatic stress,” Sherriff says. “I don’t know, is the answer.”

It is in this context that Johnson’s dismissal of her complaint as “humbug” and his assertion that “the best way to honour Jo Cox is to get Brexit done” feels so painful. “He argues that he wasn’t referring to what I said as humbug. But surely he would have sent me a little note? Or asked for a meeting? You can even do an apology that’s not really an apology: ‘I’m sorry if you were under the impression …’ But to dismiss me like some sort of hysterical female … I just think, that is the measure of the man. There are times I still can’t believe he is our prime minister.It absolutely horrifies me.”

In contrast, within the hour Jeremy Corbyn texted her personally to ask: “‘Are you OK? Do you need any support?’ He looked me up the following day and gave me a hug.” Sherriff is “not one of those who thinks everything Corbyn does is amazing”. She has twice voted against him as Labour leader. But, she says, he has “compassion, a sense of right and wrong.”

The parallels between Cox’s death and the current climate are inescapable for Sheriff. Like Cox, she is a remain-supporting MP in a leave area. Her seat is marginal and she has wrestled with this sense of being at odds with her constituents. “I wrestled with it hugely! For a while, I believed we should respect the result,” she says. She regrets that she voted to trigger article 50. “It became clear how damaging [Brexit would be]. And I just couldn’t support the shitshow. I don’t feel I am disrespecting the result by saying: ‘Put it back to the people.’”

When the general election comes, she hopes her strong work ethic will convince voters to support her.

“When Jo died,” she says, faltering, “it was almost a similar time. Tensions were very high. There was a very divisive undercurrent in the nation.” Three months earlier, the now-defunct far-right group Britain First had visited Dewsbury town centre. A few days before Sherriff and I meet, the far-right group Yorkshire Patriots demonstrated in the same place.

“I’m quite interested in the psychology of why these people feel so much hatred,” she says. “Because I was brought up in a really normal family. I’m really lucky. I had a particularly loving mum.” She mentions the many messages of support she received, along with the abuse and death threats. “One moved me to tears. It said: ‘I hope my granddaughters grow up to be like you.’”

Sherriff’s politics have very much been filtered through the women of her family. She was born near Glasgow (her mother is Glaswegian, of Lithuanian and Polish parents). But the family moved to Greater Manchester when she was a baby, then Cumbria, and later “a very modest area of Harrogate”. Her dad was a submariner in the forces, “away for months at a time” and she didn’t really know him growing up.

But her mum, Barbara, had “very strong attitudes about social justice”. She cuts stories of social injustice out of the local paper and keeps them in a folder in the living room, dying for Tory canvassers to knock on her door. She is “really feisty”. Sherriff’s grandmothers – perhaps like the one who wrote to her – were “strong influences”, and I suspect the word “strong” keeps cropping up because Sherriff sees herself that way, too.

Both grandmothers were staunch Labour. Stauuunch,” Sherriff says, as if the normal pronunciation doesn’t begin to cover the extent of it. This drives Sherriff’s work – and perhaps that of her sister Lee, too, who also stood for election in 2015, in Cumbria, but didn’t get in. Sherriff and her mother speak daily. And although Barbara, who used to work in Woolworths, “doesn’t quite grasp how much I work” – they’ll be sitting in Sherriff’s house and her mum will say: ‘Have you washed those windows?’ – she is supportive.Sherriff has a strongly autobiographical approach to public office. She joined the police after leaving school at 18 with five A-levels and an alphabet of grades - an A, B, C, D and an E (“It’s fabulous, isn’t it!”). She could have done better but boys and nightclubs were more interesting. After the police came a job in the NHS, as service manager for Wakefield community dermatology service. It’s easy to see the continuity between her personal life and her political interests.

Paula Sherriff questioning the prime minister on his use of language in the House of Commons. Photograph: HO/AFP/Getty Images

Sherriff chairs the all-party parliamentary group on women’s health. Within months of becoming an MP, she campaigned against the so-called “tampon tax”, becoming the first backbencher in parliamentary history to get an amendment to a Budget resolution adopted. It was the tampon tax that nearly derailed Johnson’s EU deal. But “it was never about the 5p in the pound,” she says (the rate to which tax on tampons was lowered as a result of her efforts). “It was about the fact that periods were considered luxuries. And it was about talking about vaginas in the House of Commons.”

Vaginas crop up quite a bit in Sherriff’s work. “I want to use my time in this place to talk about things that have previously been considered taboo,” she says. “Nothing is off-limits for me.”

Period poverty, workplace protections for women undergoing the menopause and endometriosis – “a subject close to my heart” – are all in her sights. Sherriff herself waited more than a decade to be diagnosed with “not endometriosis as such, but something very similar”, and once fainted in the supermarket with the pain of it. She dreaded being treated in the hospital where she worked because she “didn’t want to be showing my fanny to [a] guy I might see in the canteen”. But in the end, she did – and he cured her. Anyway, it wouldn’t bother her now, she says.

The treatment came at a price, however, triggering a chemical menopause when she was 37. “It was either that or a hysterectomy, and I still harboured hopes of having children,” she says. Luckily, the menopause “wasn’t too bad”.

But the children didn’t come. “Sadly. I’d have loved to,” she says, and I wonder if this is another reason that the appreciative note from an unknown grandmother was so moving. “I’d have loved a daughter. And I would have had the most feminist daughter. She would have been a little warrior. She would have been out with Extinction Rebellion.” She knows so much about this little girl; she sounds very much alive.

Sherriff looks around the room. There are trees in the atrium below, and a stall selling a House of Commons Christmas jumper. To someone whose “really poor school” never arranged trips to the Houses of Parliament, this place must have come as a huge shock. At first, she felt like an outsider. She puts on a posh voice. “‘Oh, which university did you go to? I worked as so-and-so’s spad.’”

But at 40, she was old enough to feel comfortable in herself, and in her politics. And besides, she had the advantage of a different experience. “I felt really well equipped to deal with people,” she says. “That has always been my strength.”