I had to edit this because I misread it...But i'll agree with the previous person who replied. The chances of her being pregnant from pre-ejaculate aren't quite as high as if you had ejaculated in her, but there's still the chance because pre-ejaculate can contain sperm. So i'd say, on the scale of 1 to 10, her chances are around a 5 or a 6.
Do you know if your girlfriend was due to have her period anytime soon following having sex? If she misses or has missed her period, she should take a pregnancy test...And don't worry, you can get them at dollar stores, so they won't cost you an arm and a leg. But if she's too embarrassed to buy one or what-not, then see if there's a planned parenthood clinic in or around your area. They'll do free pregnancy testing, and, if she's not pregnant, they'll give her birth control and condoms for free also.
Remember, always use protection.