Maybe this is a good time to break up with her and move on??
Infidelity is a very real problem. Some say it's a sign of character weakness, others suggest it's an ingrained part of our DNA (read: Sex at Dawn by Christopher Ryan, who argues that we are hardwired to crave novelty, which leads to infidelity in marriages).
Either way, unless you two were merely dating and not in a steady relationship, you should probably move on.
If you feel hurt because you weigh out the situation as she having done something with another man, while she is technically yours (whatever that ever truly means), realize that prior to being with you, she had been with many men (just like you have been with many women), so in reality, it's merely a betrayal of trust.
If you can't trust friends, you stop associating with them. Likewise, if you have no trust in a romantic relationship, disassociate.
All the best...
"1"