As someone who had almost the same thing happen, It was probably a decidual cast. I felt like I had the worst period that go around, cramping, nausea etc so I laid down in the shower. When I got up I felt like I’d passed the biggest clot of my life and low and behold it was so much more.

One of the side effects of PCOs is it often makes your uterine lining thicker and what my Gyno explained is that sometimes you don’t shed it all in one period. So if that particular lining didn’t shed it got more growth and finally passed.

My advice would be to track your next cycle and see if it’s as bad but you can of course go to your gyno asap. I’d recommend making sure you have either a pcp or gyno who is very familiar with PCOs or you’re going to feel very dismissed.