Ok… there are two real possibilities here:

1- she is or is looking to physically cheat. If so she isn’t very clever.

2- she is looking for validation from guys that aren’t you because she is feeling insecure, low or curious… which still really isn’t okay.

What you do is up to you. If you ask and she responds with anger… that is usually a defense mechanism and not a good sign on the fidelity front.