Menstrual Bleed. On saturday the 29th.

My period ended on Thursday the 27th. It was regular and lasted the full 7 days mine normally does. On Friday the 5th, I started bleeding again. It was much lighter and there was a lot of discharge - sometimes yellowish, sometimes white-ish/clear. This bleeding is still going on (Thursday the 11th) and is still really, really light. I am 21 years old and have never really spotted ever. I have back pains and my breasts feel sore - exactly the same symptoms I have when I am on my period. In the middle of the week that I was off my period but these symptoms had not started, I did miss two pills - but I have missed pills before (I know, not good) and nothing like this has ever happened to me. I am wondering if I am pregnant or dying or if I have an STI. In full disclosure, I had sex with my partner on Saturday the 29th.