I've had irregular bleeding my entire life, but during an especially bad episode I was once told a cyst was being "reabsorbed" and that was the cause. The blood was especially dark.

I had an IUD prior to hysterectomy. I had a lot of bleeding between periods. Like, all the time. Especially after sex or physical activity. It wasnt always heavy or bright, but it was very concerning. I was told that the endo lesions were bleeding, and that there was nowhere for the blood to go, such as menstrual blood flows. So it was randomly making it's way out and was older blood.