I slept with a guy who has a girlfriend. I didn't know!! Should I feel guilty?

So one night, I'm 18 btw, my girls and I were having a sleepover and I decided to text this guy I hadn't spoken to for a long time. He ended up inviting us to his house where we did some stuff but didn't sleep together. The next week we did have sex and then again last weekend. I thought he might have a girlfriend, but I never asked and he never told me. In fact he told me it had been some time since he had been with a girl. It wasn't even on fb. But then today somebody posted something about getting his girlfriend home safely. I feel really guilty. We are from a small town everyone knows this guy doesn't do monogamy. So I'm like doesn't his girlfriend expect this. Idk.... It's not like we are having sex again. I moved so it's out of the question. But should I or should I not feel guilty?