She could "feel" pregnant.. Some people can just tell that they are gonna get pregnant when they have sex...
But hun, I agree with Suzy, you're 14, you have A LOT of your teen years to go and to just enjoy being a kid... Don't mess that up. You know, you still have prom to look forward to and stuff like that. Do you want to miss out on that stuff because you have a baby?
Get on birth control ASAP