She should have disclosed that she had been with your best friend near the beginning of your relationship, not several months later.

You may not have felt betrayed, but i'm sure you felt blindsided.

Perhaps you are anxious about the possibility of them hooking up again?

I really don't think you having multiple partners would make any difference to the way you feel. She lost her virginity to you, so she obviously felt something for you that she didn't feel for anyone else.