help! worried about pregs possibility

hey- and my boyfriend and i were having outercourse (god this is embarrassing!!) and i, er, had undies on. that were lace. and that was it. and he had boxers on and i am pretty sure he ejaculated near my vagina so naturally, even though i am not a virgin by any means, i freak like an id**t and do all the ovulation tests and blah blah and apparently this was a couple of days after my period tapered off. i am still not exactly sure what ovulation means but apparently i was 'really fertile' around a couple of days ago, and the time i'm worrying about was about two weeks ago. i have no reason to suspect im pregnant except for the fact that life is what happens when you're looking for the cd you want at wal mart, and etc. im not going to ask you if im pregnant because that's stupid, how would you know? but i do want to know some percentages, or how much time i should spend obsessing about this before its ok to buy a fun pregnancy test