I'll bet she has butt acne.
Mama, we all get insecure in these situations.. the girl my ex cheated on me with (well, one of them) was a size 00... and it hurt and it sucked, and you know what??? I can just be secure in the knowledge that while she may rock a miniskirt... I never slept with a married man, I'm a good mother, and I was a good wife...
You can do the same.
Be strong, I know it hurts, but that pain is fleeting.