Bleeding after sex, birth control and blood clots

Hello. I'm turning 19 this month and something extremely unusual has just happened. My last period ended last Saturday, I had (protected) sex the following monday, and then started bleeding again the following wednesday. At first it was just spotting and then it got heavy. I was confused but when it started looking like I got my period again I started birth control for the first time. So now I've taken two pills (over two days, starting thursday, the day after I started bleeding) and I've just noticed that the blood is clotting. This really worries me and I have no idea what's going on. I feel I should also add in that I've been having cramps and back pain along with this. Does anyone know what might be going on? =(

(Sorry if I posted this more than once....my computer is being weird)