Does A Woman's Vagina Feel Different When A Girl Cheats?

No.  Why would it? Giving birth can cause the vagina to stretch – after all, it’s had an entire baby pass through it – but having frequent sex won’t.

Why Does My Girlfriend’s Vagina Feel Looser When We Have Sex?
She’s either more aroused, or she’s more comfortable with you.

Being nervous or tense causes the muscles in the vagina to contract, which is partly why sex is painful for rape victims. The opposite happens when a girl relaxes.

When a girl is aroused, the vaginal canal will widen in order to accommodate the penis. So, if your girlfriend is relaxed and aroused, her vagina won’t feel as tight as it would if she was tense and turned-off.

Obviously you don’t want your girlfriend to be turned-off and nervous when you’re having sex, so just see this as a sign you’re doing something right. Don’t accuse her of cheating.