The chances of her being pregnant are small. But yes, there is a very very small possibility she could be. "Pre-***" and / or "Pre-ejaculation" often happen in the early stages of a man getting "hard". This can cause pregnancy, even if you only went inside her once. Kudos to you for stopping and using a condom, at least you are trying to be responsible. I wouldn't worry about it too much, your girlfriend seems like she knows her body enough to tell - and most women do. How long a go did you have sex? That can always play a factor in it. I suggest taking a deep breath about it until you know something, you can't undo something if it's already done. Which hopefully it isn't! (Assuming you weren't trying to get her pregnant). Good luck!