What Is The Heavy Constant Bleeding In Spite Of Being On Depo Shot?

I've been on depo provera for over a year. I was on it for two years after having my daughter, switched to the pill for two years, and now i'm back on the depo. I have endometriosis and PCO. A week ago I finished taking Premarin for a vaginal tear. For two days I've been bleeding non-stop. I've been through 7 tampons today alone. The blood is thin and bright red with a few chunks. Would this be related to the depo or Premarin? Or could it be something else that I should be concerned about?