You don't need to do anything. She can't get pregnant that way. Her ovaries are not located at the opening of her vagina, they are much farther up inside her. And unless your sperm is injected up inside her, she can't get pregnant.
Yes, there is a possibility that sperm were in your pre-ejaculate, and that those little swimmers found their way inside your girlfriend's womb, and to an egg. It's not terribly *likely*, but it is possible.
Slim chance of pregnancy and even slimmer chance if she took the 72 hour pill.
Like stated above, best idea to wear a condom whether you are having sex or just other sexual activities - protects against pregnancy and also just as importantly - STDS.
It's possible to become pregnant this way, though less likely than with actual intercourse. Suggest that if pregnancy is undesired at this time you consider using a condom regardless of whether there is penetration.