unusual issues

So I was at a party last night, sober as all hell (I promise, I was the sober cab) and I was smoking my normal cigarette, talking with some good friends of mine and out of the blue I felt like I needed to throw up. So I did all over the yard. (Classic, I know) My boyfriend and I have always had unprotected sex.. (stupid, I know) and that weekend he didnt pull out.. I didn't think I was pregnant, however my period as usual is off. So I wasnt worried at all. Well I went to walmart got a pregnancy test and It came back negative. So I also bought plan b just to be double sure. Has this ever happened to anyone else?? If so, do you happen to know why?