Am I pregnant or not?

I'm on birth control but i started taking antibiotics and they say when you take that it will screw up your birth control. My boyfriend and I have been having unprotected sex and i usually start my period on the last blue pill. Well i didn't start. 4 days went by and i still didn't start until today. Now either it is implantation or its actually my period, but Ive been feeling this tiny urge to throw up but i don't. I have honestly NEVER been late for my period. Ive always been early. So I'm wondering why i missed 4 days. Just all of this has confused me. So does anyone have advice for me??? Also i have this cramps that are unbelievable. I just want to stop what im doing and lay down. My cramps aren't usually that bad.

ok, i'm no expert on pregnancy by any means, so you can choose to listen to my advice or not. i would suggest taking a pregnancy test to find out for sure, because really, i don't know of any other way to find out other than waiting for it to show up on its own. or you can go see a doctor; i'd assume they'd be pretty accurate with being able to tell you about it. that's really the best that i can come up with. good luck!