Idk if this helps, but I have PMDD, which is kind of the same, but before my period instead. It's definitely because of hormones, so you may have to check with your doctor. I know someone with PMDD is on a certain birth control pill that helps, but your doctor would know more.

I can relate to this. I think, for me, it's because being more aware of my body is triggering. I can also imagine that the way hormonal changes affect emotions could play a part. I find that tracking and recording these symptoms helps because I feel less surprised and out of control when it happens again. Also allowing myself to lean more heavily on my usual strategies, and reminding myself that it is only for a few days.

I have PTSD to the pain caused by my endometriosis and PCOS, the pain triggers flashbacks and actually have periods is worse for me so I have progesterone tablets to stop my periods. If you get particularly heavy or painful periods it could be that you were experiencing that when your trauma source occurred and so it is a trigger in of itself. The amount of people who don't realise they have an abnormal amount of flow or pain is crazy, and so it's worthwhile speaking to the doctor about a possibility of an underlying issue with periods because management of the periods themselves could help with the issues with the PTSD as well.