Period problems.

I'm 24 and had my baby girl in April. After having her I got on the mini pill because i was breast feeding, it caused me to have 2 periods a month. About 2/3 months ago I quit the pill, had a couple regular periods but now im on on my period and its been going for 3 weeks. I dont know whats going on, ive never had this problem before. Im also having a lot of lower back pain. Anyone been through this before?