Question..opinions needed.

So I started my period Monday, yesterday it started to lighten up and then today it was almost completely gone I got home about an hour ago and just went to the bathroom and I had dark red blood in my panties, not a lot but enough that I decided to put a panty liner on. Me and my SO had sex last night because it was pretty much gone by time 12 midnight came, and he did cum in me. then i was spotting after and this morning but it was dark brown and VERY light spotting. Soooo why am I bleeding like this again?