sperm might have gotten onto her pants. pregnancy?

Last night my girlfriend and I were messing around

I had just my underpants on, she had underpants and jeans on , we dry humped a tiny bit moving up and down (not much at all) but I am really concerned some of my sperm might have gotten onto her pants.. why?
1. I masturbated around 15 mins before, could some of the sperm have gone on my underpants? although I couldnt see anything on my underpants and when I did masturbate I ejaculated into a tissue...
2. Could the sperm have gone through my underpants and through her clothes?

Her jeans didnt fall down or anything, they were lose but they didnt fall down as to expose her pants and vaginal area...
so what are the chances pregnancy has occured? and should we consider a contraceptive pill?