Is my girlfriend cheating on me?

I suspect my girlfriend of 5 years might be cheating on me. I came home one night and we had intercourse and my penis slipped in with ease. Usually it takes a minute or two of intercourse before her vagina loosens up but this time in went right in. I don’t want to jump to any conclusions. She has 3 kids from her previous relationship. So my question is can the vagina loosen without any physical penetration?