do i have an std?-or-am i pregnant?

:?I am really confused and scared right now. i just recently, like 3 days ago, had unprotected sex with my boyfriend, and whenever my boyfriend and i have sex, he always pulls out before he cums. well the next day i started to have brownish dischange show up in my panties and it freaked me out, but i didnt know what to do, so i put a tampon in. it has continued through today. to give you a little more background...i am 17-years-old and have been taking ortho-tricyclinlo (the birth control) for over a year now. i have never missed a period, i just started one late because i was very stressed from school. i have missed 1 pill before, but this month i have missed 2 pills, like i have had to take 2 pills on 2 different days to make up for missing them, because i had prom one night, so i took 2 the next night, and then we had a slumber-party so i missed another the next week, so i had to take 2 again the following day. with all this said, im thinking the missing of pills may have effected my period but im not sure. i was reading other questions on here and someone mentioned that these symptoms could be an std, so im like really, really, really scared and stressed...do i have an std? or even worse am i pregnant?:? thanks for your response in advance!:)