lower abdominal pain and missed period

I am 17 years old and had sex 3 1 /2 weeks ago ever since then i have been having lower abdominal pains its starts with the left side and shoots across to the right side. i also have a tighting not really pain but discomfort in the upper stomach area. ive been feeling like im going to throw up and only have once and i been having sever headaches. my dad thinks that its because i havent been on my birthcontrol for a while almost 8 months to be exact due to my polycystic ovaries but i havent missed a period for almost 8 months since ive been off. even after i started taking them im still haveing the same pain what do i do somebody please help me