Hi op. So sorry you are feeling ill.
I am not medically trained whatsoever but If you are worried, I would seek medical advice or call 111. I know you say that the scan showed cysts in your womb, not your ovaries, but maybe one has grown there since the scan? The pain you describe does sound like that I used to get in my ovary which was horrendous. Also ovarian cysts can burst or twist which is a medical emergency.
https://www.nhs.uk/conditions/ovarian-cyst/
Also, have you considered appendicitis; although it’s less likely I suppose that it would start exactly at the same time as your period?
Generally speaking, I also used to vomit from the severity of the pain of my ordinary periods, the cramping I mean, and I believe that is due to prostaglandins which causes the cramping?
I suffered horribly with nausea and vomiting for years, associated with severe period pain and no doctors took it seriously!
I had fibroids and large ovarian cysts and it got so bad during menopause that I would vomit on a regular basis after I had eaten, or early in the morning, whether I was on my period or not!
A gynaecologist and the surgeon who eventually performed a hysterectomy, told me repeatedly that my digestive problems (regular vomiting and loose bowels) were nothing to do with my gynae issues but the minute my uterus and ovaries were removed; they all stopped immediately!
Hope you feel better soon! Don’t let them fob you off with “it’s just women’s issues”.