Lower backache, lower stomach ache, dark nipples, generally feeling ill?

I was on birth control for 7 months, and then took a week of antibiotics which obviously stopped the effects of the pill. During this week I had sex, unprotected. That was exactly a week ago, and now I'm experiencing sharp yet aching pain in my lower back, a burning ache in the lower area of my stomach accompanied with a few cramps on and off. I put the cramps down to gas which I've been having trouble with for the past two days and I don't know if gas is related to pregnancy at all? I also just feel generally down, and emotional kind of like PMS except I'm not due on for another three weeks. I was under the impression that if I was pregnant, signs wouldn't be clear to see right now o.O ! I must admit my boyfriend and I are quite scared, we are only 17 :'( I have no idea if what I described is an indication that I'm pregnant but until I am able to take a test I need some sort of guidance :-(