It's highly unlikely. Sperm can't usually swim through clothes. Basically, the only way you could pregnant that way is if your underwear were soaked with semen. Sperm needs to be wet to stay alive. If wet semen isn't near your vagina, then you're probably not getting pregnant.

You can check out these sites for more information on common sex, pregnancy, and puberty questions: http://www.coolnurse.com/sex_faqs_index.ht m
http://ehealthforum.com/health/topic76600. html