For pregnancy to occur, the event you describe must happen right around the time your girlfriend ovulates, which is approximately two weeks after her period ends. If this didn't happen during those 24 hours of ovulation plus or minus a day or two, then you have low odds of getting her pregnant. This is something usually overlooked by young sex partners. A woman can only get pregnant when she is ovulating and ovulation only happens during one day, once a month.
Sperm can live for a few days in ideal conditions, so if you have sex a few days before ovulation (or if you get some sperm into her with your fingers), then you can get her pregnant. It is possible for sperm to live on your fingers, especially for a few minutes, then be transferred to her vagina so I'd say there is a small chance you got her pregnant, but it would be pretty small. Nobody can give you the exact odds. Just wait and see.
Alternatively, if you don't want to wait, you can always buy a pregnancy test kit at a drug store for a few dollars and check it for sure. In any case the odds are pretty low, but you never know, it could happen. It certainly has happened to others many times before you. One thought that comes to mind: if that sperm had the guts and strength to get into your "pre-cum", survive on your fingers, and swim all the way up your girlfriend's vagina to meet an egg, then that's going to be some spunky kid you made!! Some lively soul who really wants to arrive on this Earth big time!
making a small donation to science.ca.