Help! 16yr old - pregnant???

I've been on the pill for 2 1/2 months and three weeks ago I had sex with my boyfriend. At the beginng of the intercourse, he didn't put the condom on put he only penetrated twice (just to feel what it felt like), and since then we had twice (w/ condom). Last week I was constipated for 3 days this week for 4 days. The point is that I haven't changed anything in my diet. Also, I had two huge pimples this weekend. I'm 16 and I know these symptoms don't indicate that I'm pregnant, but I'm DESPERATE! Should I be worried?