When Cass Clemmer first got their period as a 15-year-old struggling to understand their gender and sexual identity, it felt like the end of freedom.

“I remember, very vividly, the internal chaos that plagued my body the minute I saw those brown stains on my underwear,” Clemmer, 24, tells PEOPLE. “There was a very clear switch in my brain that told me this was the moment I had to start thinking about makeup, boobs, and boys. My 15-year-old mind was riddled with the stereotypes around femininity, but I had grown up knowing that periods were so tied to ‘becoming a woman’ that I almost would wish and pray to God that I would never get my first period, just so that I could continue just being me instead of feeling forced into a box I didn’t feel comfortable in.”

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The reproductive health activist and artist still feels that battle today when they get their period every month. As someone who prefers to “live in between and around the two socially constructed categories of ‘man’ and ‘woman,’ ” and so prefers the pronouns “they” and “them,” Clemmer struggles with the changes that come with each cycle.

“I usually have to swap out my favorite binder for looser sports bras that don’t compress my chest as well and lead to a lot of misgendering and gender dysphoria during my cycle,” Clemmer says. “I’ve tried going into men’s restrooms while bleeding, but when I realized how out of place the sound of a tampon or pad wrapper is between the stalls and the consequent risk to my own physical safety, I knew that I would be better off just using the women’s restrooms during my cycle.”

For Clemmer and other people who identify outside of their birth gender, “being on your period is an automatic neon sign to the world that you were assigned female at birth, and that alone can be terrifying,” Clemmer says. Which is part of why they decided to share a photo of them freebleeding — to show that it’s not just women who menstruate.

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“I really hope that people see my post and start to consider that not all people who menstruate are women and not all women menstruate, so that we can start to collectively move away from the harmful tendency to equate periods with womanhood and femininity,” Clemmer says. “The presumption that everyone who gets their period is a woman or a girl serves as an erasure of the identities and experiences of an already marginalized community.”

And there isn’t a simple fix, as hormone treatments don’t always stop a person’s cycle — plus many trans people prefer not to take them.

“I do not speak for all trans and/or nonbinary folks, but not all of us choose to take or even have access to hormone treatments, so when people tell me to ‘just start taking T’ to end my period, they often don’t realize it’s a lot more complicated than that,” Clemmer says.

Clemmer’s post drew a mixed response — “ranging from support and sympathy to criticism and downright cruelty” — with the scale leaning towards the latter.

“Unfortunately, the transphobic comments far outweigh the positive ones and include everything from people calling for me to be put in a padded room to violent threats and claims that I am a child abuser for posting a photo of myself freebleeding,” Clemmer says.

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But they say that the negative reactions are necessary to deal with — as terrified as they were to share their story — because it’s eliciting a response from people who were just unaware.