If you're going to worry yourself about that, at least pull out next time. Withdrawing is only about 15% effective but better than staying in for the ride.
Things to think about:
1. If you want to make this a habit, there's this cool thing called the "pill"
2. As a general rule of thumb, girls are "safe" for the first 14 days following their cycle. But that doesn't mean it's impossible for bad things to happen.
3. If you're REALLY worried, go with her to a clinic and get the morning after pill. It's good for 72 hours after the incident.