What is this?

Hi! I'm not sure exactly which category this should go in, so I hope I'm in the right place. I gave birth 3 months ago and started the pill 10 days after delivery (one that's safe for breastfeeding). After 6 weeks, I stopped breastfeeding and switched back to my regular pill. At that point, I'd had my period for 10 days. Now I'm in my pill-free week again, and yesterday I had a bit of brown discharge, then bright red blood in the evening. I thought nothing was wrong... But this morning at 6 AM, I woke up with terrible abdominal pain and couldn't sleep anymore. So I went to the bathroom and was bleeding heavily, with quite a few clots. The heavy bleeding lasted until about 1 AM, with waves of abdominal pain throughout. Now the bleeding has reduced quite a bit, and I still have some dull pain in my abdomen that radiates a bit into my back. Does anyone know what this could be? Best wishes!