Has anyone else ever experienced this?

I am 3 months postpartum, since having my baby I have gone back onto the same contraceptive pill that I was on previous to getting pregnant. Prior to being pregnant, my periods were very irregular and light. Since having a baby I have had two periods which have been heavy and lasting for over a week. I came off my period last Wednesday and had sex on Sunday, after sex I felt a little bit sore and when wiping I saw there was what looked like old blood. I thought maybe having sex may have irritated the inside but now I am bleeding more like I’m having another period. I was off my period for 3 days. Is it possible to have another period that close together? Or has having sex had some kind of impact?