It is a truism that the political landscape is shifting at unprecedented speed, but in the case of one MP first elected in May 2015, the past 14 months have been outlandishly surreal. The earliest indication of how much has changed comes when I cannot even find her.

The address is an office block in Bristol, but I search the building in vain for any clue as to the whereabouts of the constituency HQ. After a while, an aide finds me wandering about and steers me through an unmarked door to meet the smiley but apologetic MP. This is due to security advice following Jo Cox’s murder, she explains: until an intercom and lock have been fitted to the building’s entrance, she dare not put up signs announcing her presence. “I know,” she says, seeing my expression. “Isn’t that sad?” Although briskly upbeat throughout our meeting, Thangam Debbonaire has a lot to feel sad about.

When the former cellist overturned a Lib Dem majority of 11,000 to win Bristol West for Labour last year, she was euphoric. Ten days later, she discovered a lump in her breast so large she could actually see it. She told no one, not even her husband, and in a frenzy hired premises and staff, delivered her maiden speech, took part in her first debate and recruited parliamentary assistants. A month later, doctors confirmed she had cancer. “For about 20 minutes, I did think about not telling anybody. Then I realised it was impossible,” she says.

Debbonaire informed the Whips’ Office that she faced nine months of chemotherapy, surgery and radiation. “I could only manage two priorities: constituency and cancer,” she says. She decided to work in the constituency 50% of the time, and told the office she would not be back in Westminster before the following spring. They were “fantastic”, Debbonaire says, and arranged for her to be paired for the duration.

Jeremy Corbyn on a visit to Bristol last year with Thangam Debbonaire and Labour mayoral candidate Marvin Rees (now mayor). Photograph: Chapman/LNP/Rex/Shutterstock

The 49-year-old used to work in the voluntary sector on domestic violence, and describes herself as a “northern European socialist – a democratic socialist”. She talks proudly about the achievements of the last Labour government, but declines to characterise herself as a Blairite or a Brownite, and says: “I’m sort of pragmatic middle left. Capitalism, when it’s unfettered, is unjust and oppressive. I like fettered capitalism.” I think it’s fair to say she would never have been a Corbynite, and in the midst of her chemotherapy, last summer’s leadership election mostly passed her by. Before her illness had been diagnosed, however, she says she had a “weird” conversation with Jeremy Corbyn.

He was trying to get on to the ballot paper, and Debbonaire’s inbox was being “spammed” by his supporters – many in her constituency – urging her to nominate him. “But he didn’t get in touch with me. Other people standing for the leadership got hold of me. That’s the way you normally win elections, isn’t it?” Instead, she contacted him, left her numbers, and “eventually he rang. It wasn’t a comfortable conversation.”

Debbonaire asked him what he thought was good about the last Labour government, and says that after some thought he came up with the Human Rights Act and the Equalities Act. Pressed for more, she says he offered something vague about domestic violence. “Now that to me is not someone who wanted to win that election. You only had to Google me to see that my entire career had been in domestic violence, and then he could have said something a bit better, because that’s how you win people over. Instead he said: ‘This is starting to feel like being on the Today programme.’ And I said: ‘Well, you will have to be on the Today programme if you get elected.’

“At no point in the conversation did he even ask me to nominate him. It was just so odd.” How did she feel when Corbyn was elected leader? “My feeling was that it was going to be tricky. It seemed to me that Jeremy’s comfort zone was not in parliament with other MPs, and I thought: ‘How is he going to run the shop?’”

She discovered how on 14 January. “I started radiotherapy that morning, came to work, and a member of my staff said: ‘There are journalists ringing, and it’s on Twitter that you’re the shadow arts and culture minister.’ I said: ‘Don’t be silly.’” She was about to issue a statement denying this when an aide suggested calling the Whips’ Office first, just to check. “That’s when I found out. They asked: ‘Hasn’t Jeremy asked you? He was supposed to have rung you.’ I said: ‘I think that’s a conversation I would have remembered.’”

Thangam Debbonaire at a hustings debate on education with candidates for Bristol West in April 2015. Photograph: Adrian Sherratt

Had Corbyn called, she says she would have told him: “That’s really flattering and I’m really honoured, but I’m not through treatment.” It would have been her dream job were she well, but under the circumstances it felt impossible. Debbonaire didn’t know what to do. She called Maria Eagle, the shadow secretary of state, and said she wanted to “tough it out and try and do the job”.

For six weeks, Debbonaire did her best, focusing her brief on local arts organisations in Bristol. Every week, she rang the shadow secretary of state, and Eagle kept saying Corbyn was going to call her. “I was beginning to wonder: ‘Why is Maria being so funny?’ I was beginning to get the message that something was wrong.” When he finally phoned, nothing had prepared her for what he had to say.

Debbonaire was not, she discovered, the shadow minister for arts and culture after all. Corbyn had appointed Chi Onwurah to shadow the secretary for culture, media and sport back in September, and then given half her job to Debbonaire in January by mistake. “I said: ‘But your team press-released that I’m shadow arts and culture minister.’ And he said: ‘Well, that’s Chi’s job.’” Twenty four hours after appointing Debbonaire, when his error came to light, Corbyn had reassigned the brief to Onwurah. “I asked why he hadn’t taken the trouble to tell me, and he said: ‘I didn’t want to bother you.’” For six weeks, while undergoing daily radiotherapy, Debbonaire had been left in the dark to do someone else’s job.

Corbyn agreed to a meeting, she says, but then “stonewalled” requests for an appointment “until my assistant marched over to his office and said: ‘My member’s had cancer, she’s come back to do this job, she needs to see you.’” When they met, Debbonaire asked how she was supposed to explain why she wasn’t the minister. “Do I say you made a cock-up? Or do I lie and say I’ve changed my mind and I don’t want to serve, and your supporters give me a really hard time.” She worried she would incur the wrath of Corbynistas. “I had seen what happened to Catherine McKinnell when she had resigned. She got a load of abuse from them. I thought: ‘I don’t need the grief.’”

Corbyn, she says, could offer neither an explanation nor a solution, and eventually an aide was dispatched to summon Eagle, who told him: “Well, Jeremy, you are the leader, if you want to give Thangam the job back, you can.” Debbonaire says she left “none the wiser”, but in due course heard from the leader’s office that she had been reinstated.

Debbonaire insists she had no intention of resigning until Corbyn appeared on television the morning after the EU referendum and called for article 50 to be triggered. “You cannot serve on the frontbench if you oppose the leader on something so fundamental.” She insists she played no part in any orchestrated coup, but admits “it would have been hard to resign alone”. After standing down and voting for the no-confidence motion, she faced her local party AGM.

Bristol West is a young, well-educated urban constituency, “in many ways, classic Corbynista”, she says. Membership had swelled with his supporters, “and many of them turned up to the meeting, really hardcore Corbyn supporters, and yelled and shouted that I’m a traitor”. Some shouted at her about her parliamentary voting record, either unaware or indifferent to her cancer. A torrent of online abuse was unleashed along the lines, she says, of “fuck you fuck you fuck you, scab scab scab scab scab scab”.

Corbyn himself has admitted that the fiasco “was not well handled”, telling Newsnight’s Evan Davis that he was “aware that she was receiving very serious cancer treatment and didn’t want to disturb her”. He said: “Unfortunately, my wish to appoint her as one of our arts spokespersons was informed to her when it shouldn’t have been. I wanted to leave it until her treatment was over and it was. She wasn’t sacked … when she returned after her treatment I had a very long conversation with her, and of course I apologised to her for that.”

Debbonaire’s manner oscillates between combative and conciliatory. For now she has withdrawn from Twitter, but says most of her members have been “wholly supportive”. She points out that some of her critics may be unaware of Westminster’s pairing system, and don’t realise her parliamentary absence was accounted for. She insists she “really, really didn’t want” to reveal the shambles of her appointment, and only did so in a statement earlier this month “so that people could know what Corbyn’s leadership is really like”. But, while Corbyn has apologised, when I ask how members have responded, she looks tired.

“Some people have said that I’m lying. That I’ve made it up.” She gives a weary laugh. “But you couldn’t make it up, could you?”