It sounds a lot like fibroids unfortunately. But you're so young, I think it is unusual to get them so young. I think you definitely need to see a gyne to get checked for fibroids cause you're going to want to take care of those before proceeding with the transition. Unfortunately, with fibroids, once they start gushing like that, it doesn't take long before you become severely anemic from blood loss. Trust me, it's better to handle it sooner rather than later.
Hi there. Welcome. You absolutely should see a doctor. That is well outside normal and something a doctor needs to investigate. A doctor can go over medications and such that may be necessary during your transition so speak openly about that. However, there is something going on. Fibroids perhaps. No one on the internet can really help you with that as this requires a physical examination and most likely blood work. Good luck and come back and tell us what the doctor says, okay?