What Causes Abnormal Prolonged Vaginal Bleeding After Sexual Intercourse?

My periods have been heavy for the last 8ish months. I'm now 37 years old. They always start heavy, bad cramps for the first 2 days with heavy bleeding, the next 2 days are normal with no cramps and then I'm done. This time, I had sex Tuesday night and started bleeding during. The next day I didn't bleed at all. I got my period on Thursday and it was lighter than normal Thursday and Friday but I still had the bad cramps. Saturday and Sunday I had heavy bleeding with a lot of clots and still had the cramps. Monday I barely bled at all but still had cramps and I was really dizzy. Tuesday I didn't bleed at all but them bled Tuesday night. Today I didn't bleed at all, but I felt something a little bit ago. I went to the bathroom and my panties were full of dark blood and I'm cramping again. Also, my lower back has been killing me all day today.