Can I suggest you both stop stressing about what if she is, and go buy a pregnancy test to find out for sure? A girl can get pregnant any time of the month so there is not formula you can use to predict if she got pregnant or not. When you had unprotected sex and the position you did it in makes no difference. Hell in my area 99 cent stores sell 2 pregnancy tests for 99 cents, and if your gonna rush out and buy one why not pick up some condoms just in case you guys got lucky this time. No wait, go buy the designer condoms, look for RIBBED or some crap. Trust me, I got some free stuff at the LA EROTIC show last year, and those ribbed bad boys cost more but less then having a baby or the other alternative.

Don't guess, be an adult and go get a test.