Me and my girlfriend's sister had sex at her apartment when we were drinking. Could she now be pregnant?

Hi. Ok, not long ago, me and my girlfriend's sister met up by chance at a grocery store. To make a long story short, we went back to her place. Her and her sister don't get along well. They haven't since childhood. So, her and I are alone drinking beer and then she starts kissing me and feeling my penis getting bigger. Soon after, we're both naked and in bed. I asked her if she had a condom for me but she told me not to worry because she was on the Pill. She was so much hotter and sexier than my girlfriend is and I accidentally came in her doggystyle. I know I did because I saw my stuff dripping down her legs when we were done. Afterwards, we fell asleep. I got up the next day and left. I'm worried now she could be preggo and my girlfriend will find out about all this. I've seen her sister a couple of times since then but we both acted like nothing ever happened. But her sister does look like she's gained some weight since we were together. She's got a little belly now. How do I deal with this situation? And advice would be nice. Thanks.