Period issues since my son was born!

I was wondering if anyone else has had this problem, I had my son last year in October and got the bc shot the day we come home from the hospital but i never went for the shot again after (3 months later) and now my menstruation is messed up an out of wack! Instead of a normal 5 day like before pregnancy now I'm lucky to not have one 5 days a month and it's been a year since so every month im menstrating 20-25 days a Month and I've never been this irregular I have 2 other children and this never happens with them after birth! Anyone else have this problem or something similar and is there something wrong????