Found out my girlfriend hooked up with a guy while we were in the talking. How should i feel about it?

I recently found out my gf (20) hooked up with a guy while her and me were getting serious and in the talking stage. We eventually ended up dating probably 4 weeks after she had been with this guy she met on tinder.

I don’t know how i should feel about this because she lied to me and said the last time she ever did anything with a guy was 3 months before we ever met. But turns out it was a week before we had hung out for the first time and by that time we have been texting for about a month now.

I feel betrayed because I was only talking to her at the time but she was still getting with other guys. Should i be upset or am i over reacting? Is there any questions I should ask her or let it go?