Me and my girlfriend had sex yesterday once with a condom, and then about an hour later without one, but I pulled out well before I came, so I know for sure I didn't cum inside of her. I'm sure I pre-came inside of her though and I'm worried about pregnancy. She was on the pill for about a month and the switched the birth control brand, without missing a day of the pill, and 4 days later after switching the pill brand, this happened. Should I be worried?