I feel like I'm pregnant.I'd feel better if I had someone's input on this

So I've been on birth control steady since August of 2012...

I had unprotected sex about two months ago, December 8th or 9th to be exact. I got my period at my normal time in late December which was about a two weeks after intercourse. I then got my period at my regular time in late January, which is a little strange because since I'm on the pill Im supposed to get my period every other month. But the fact that I had my period assured me I wasn't pregnant. I started to become a little nervous though, since Ive been gaining weight, so I took a pregnancy test about a month later, January 11th. The results were negative, but I still feel like somethings not right. So again, I bought another two tests over the weekend and the results yet again came back negative. Im sure it's accurate, but you never know for sure...should I see a doctor or should I be okay?

I'm only 17 years old... I dont wanna wait until it's too late. I guess I'll see when I get my period again, since I haven't missed, however I'm soooo confused.... please help :/