In Annie Hall, Woody Allen says that a relationship is like a shark--it has to keep moving or else it dies. Maybe she didn't think things are moving.
More importantly, it doesn't matter if someone else had her body, it is you who she says has her heart. If you really honestly love the other person, it doesn't matter if they were with others while you were apart. You don't judge them for it, but try to understand them. She broke no promise to you, since you were apart then.
Appreciate that her experience made her realize how much she cares for you, and if you still love her, don't let the fact that she had sex with someone else stop you from inviting her back into your life.
