Hi,
If she wasn't naked, her clothes would act as a barrier against semen, so I don't think you have anything to worry about at this point. Semen has to come in direct contact with the vagina in order for a woman to become pregnant. Both of you should seriously consider using condoms and birth control from now on. If you and your gf still believe there is a risk of pregnancy, she should take a pregnancy test soon for a more accurate answer.
Good Luck