Are you still breastfeeding? If you are breastfeeding still it could also be due to that. It is not uncommon for your first few cycles after AF returns to be extremely heavy and last quite a long time (like it was after you gave birth). Is it possible that your last AFs were just break through bleeding and not actually AF at all?

I was just giving some further ideas, but if you are concerned about it, you should see your GP.