What Could Heavy And Liquidy Menstrual Bleeding Suggest?

My period used to be heavy,thick, dark red ,full of clots & last 5 days approx. The last two months, my period is still heavy but more liquidy and a lighter colour red- there is no pain/cramps. Last month it lasted so long, after 11 days, I went to my doctor-she put me on provera to stop the bleeding and also some iron tablets. I did a pap smear, full lab tests for hormone,vd,stds- everything was ok. What could cause this change in my period?