What Causes Irregular Heavy Periods In Spite Of Medication?

I am 30 years old and have had my period for 2 months now. My doctor had proscribed provera to help about 3 weeks ago, which only lasted for 3 days then I started back up again. Now my periods are heavy with huge clots (I normally have regular flowing periods, aprox. the same time every month). What could the problem be?