What Causes Heavy Menstrual Bleeding?

I am 33yrs old, two kids and have been on birth control for the last two years. I was on orthotricyclen and missed a pill back in June. I have had irregular periods since. A midwife friend of mine suggested the Depo to lessen the bleeding but I've been bleeding pretty much nonstop since Sept 9th when I got it. I have not had any problems with birthcontrol up until this point. I've never had an abnormal pap. Is this most likely hormonal? Any chance I'll regulate when the shot is "out of my system" in a few more weeks?