What should you do:
First you should find out whether you were actually cheated on. Here is an article about cheating spouse signs.
Find out what exactly happened, and talk to your partner about it. Was your partner under the influence of alcohol?
Find out how likely it is that your partner will cheat on you again. Cheaters who confess without having been caught are unlikely to repeat their mistake. However, if your partner has a history of cheating on you, don’t waste your time.
Make sure you can live with the knowledge of your lover’s betrayal.
You have to be willing to forgive. Don’t turn your relationship into a cycle of mistrust and revenge.
Be sure that your partner understands what kind of damage he/she has committed.