Pregnant or STD??

Hey guys. 
I had what seemed like my period a week ago. It was more light than usual, and the last day more like vaginal discharge with blood. It only lasted four days. It came sooner than expected (37 days) which is short for me, because my period is very irregular (sometimes 46 days). 
Now I am feeling pain around my vagina opening when I am tightening my vaginal muscles, and I have been feeling dizzy and my arms are shaking sometimes. I can't physically see if anything is wrong. 
Does anyone know what it is? I am 16 btw. I don't know how I could have gotten pregnant since the last time I had sex, was 50 days ago, and I got my normal period 2 days afterwards.