What Is The Cause Of Heavy Bleeding?

I am 37 years old and I am still on my menstrual (11 days) and it is heavy bleeding along with clots. I have been to the doctor and this has been going on since November 2013 which last year my menstrual lasted 18 days. Why am I still bleeding when my menstrual use to last only 3 days and not heavy. Am I going through pre-menopause? I am not able to have any more kids due to my tubes being clamped and I only have 1 ovary due to 4 years ago I had a tubal pregnancy that resulted in me having surgery. Is there anything that I can take to stop this bleeding? Please help.