I haven't experienced it, but there are a few plausible explanations behind it. People who dismiss it out of hand just aren't very creative thinkers. There are a few plausible hypotheses.

One I've heard. Some searching suggests even cis men experience a slight monthly hormone cycle. It's not as noticeable as that of cis women, as the hormone fluctuations aren't as large and there's no externally visible symptoms. So whatever mechanism that causes cis men to have a monthly hormone cycle, whatever signal is triggering that cycle, would likely be present in trans women. And when a trans woman goes on estrogen, that monthly signal could trigger symptoms more familiar with the female cycle. Things like cramping and diarrhea. As far as why only some trans women experience this? Only some cis women get noticeable cramping or period shits. Plenty of cis women have periods largely uneventful aside from the bleeding. If your genetics have a predisposition to period cramping and digestive issues, then that will be activated when you switch to an estrogen-dominant system.

The key to all of this is that it wouldn't actually be the hormones themselves triggering the symptoms. (As trans women's hormone levels are usually pretty constant, any monthly pattern would have to be triggered by the mechanisms that cause hormones to vary monthly in cis men or cis women in the first place.)

A second possibility I don't see discussed is that it may simply be endometriosis. Trans women don't have uteruses, but we can grow uterine tissue in certain cases. It hasn't been studied really at all, so we have no idea of the prevalence of it, but case reports do exist. There's been more but still limited information on endometriosis among cisgender men who underwent long-term estrogen therapy for prostate cancer. But there really hasn't been any extensive study of the prevalence. For all we know the majority of trans women who've been on long-term HRT could be walking around with some amount of unnoticed uterine tissue. It's never been studied; we have no idea how common it is.

Really a lot of this could be explained by trans women getting endometriosis. At first it seems absurd, but it really shouldn't be. Trans men get bottom growth. Testosterone doesn't cause a clitoris to grow into a full-sized penis, but it does grow and develop certain penile tissues and penile characteristics. Why shouldn't trans women get a version of bottom growth? Bottom growth is noticeable for trans men because male reproductive organs are external to the body. The trans woman equivalent of bottom growth would be entirely internal.

What would really be happening is that a stunted and very limited version of the uterine development that occurs in female fetuses in utero. At the locations where the uterus grows, a limited amount of uterine tissue would grow instead. And if a trans woman grows uterine tissue growing around her colon or intestines, having that tissue trigger period-like symptoms suddenly doesn't seem so absurd.

I have a period tracking app, and log when I randomly burst into tears over every stupid thing for a day or two. It's about once every four weeks.

I don't cycle my HRT or anything.

Varies from person to person.

You get PMS not menstruation, so all the “fun” minus the bleeding. I get some yucky physical stuff and super sad, but on the plus side my boob game is at its peak for a few days.

I haven't had physical pains, but you absolutely will get wrecked by it when it hits the first few times. Feels like every trigger for negative emotion got dialed up to 11 and you can cry at the drop of a hat. It sucks, and you get to look forward to that every month.

Frfr