What were you taking in between if anything, I had this happen, but I didn't stop bleeding for over six months, it turned out that I had gone onto the Depo too quickly after giving birth and my womb had not had time to get rid of all its gunk, I had to have a d&c to clean out. That sorted it.
When I first went onto the pill, I bled for a month... I would guess that you have the same thing. I wouold go to the doctors to have it checked out however if you are that unsure.