If you simply asked her about it, and she broke down crying, leave her ass. I don't know ONE woman who will cry unless she's guilty...if they are asked and it isn't true, they tell you you're trippin, and maybe even say something like "I can't believe you think that bullshit", and then get on with the day. But, if you put it simply, in question form, without yelling or anything, and she cried, she's probably doin' SOMETHING!!
If a girl does get pounded a lot, her shit will stretch out and become somewhat looser after a while. BUT, the shit should not stretch beyond the size of your dick unless she's fuckin' somebody bigger.
I've had my chick for almost 6 years, and when I tag it, it's always form fitting...LOL!!