Even if you don't notice it or feel it, "pre-cum" or pre-ejaculate is present upon arousal. Pre-ejaculate can contain sperm. It only takes one sperm to cause a pregnancy, and the vagina is a fertile environment that does its best to get that sperm up to the egg.
If your girlfriend does not wish to become pregnant, and it is 72 hours or less since the event, you and she may seek emergency contraception. To find a provider near you, please call 1-888-NOT-2-LATE.
STDs are also a concern in this matter, since you do not indicate that you used any barrier method of protection. Both you and she need to go in for full STD screenings and possible treatment.
For more information on pregnancy risk, see What's the Risk?
For more information on STDs, see The Infection Section
"Ooh, I'll settle down with some old story/About a boy who's just like me/Thought there was love in everything and everyone, you're so naive!"-Belle and Sebastian, "Get Me Away From Here I'm Dying"