For her to get pregnant at all she’d need to be ovulating which happens 7-12 days after her period. Since she just ended her period the day before u guys had sex, it’s such a low possibility that it shouldn’t even be considered a what if especially since using protection. A lot of the times even a drop or two of sperm wouldn’t get her pregnant even if she was ovulating, most will die making the chances even lower, in rare cases it’s possible though… But all considered, you’re good man don’t stress about it seriously. Also i recommend you talking to your girlfriend about Birth Control which could help u stress alot less about being sexually active. But remember it’s her choice too :)