Did you both have your clothes on? If you did, and none of the sperm actually got on you or especially on your skin "down there" the chances of you getting pregnant are very small. If however you both had your clothes off and some sperm DID happen to get in the vicinity of your vagina, then there's much bigger chance that you could, in fact, have gotten pregnant from it. But there's a lot of factors to consider such as your period.
Keep an eye on things, you should be pretty aware of your body's monthly funtions. If you start to get really worried in a few days or you miss a period, get it checked out! Next time tell your bf to be a bit more careful, keep your pants on, or keep your baby-making parts away from each other. =P
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