You are upset and hurt that's normal when it comes to matter of the heart. Just consider yourself lucky, look at it as a blessing that you are finding out now rather than later. Most marriage that last is because both people love and respect each other, there was no Fake, No pretense, what you see is what you get. I've been down that road my friend, the more you keep it inside, the more it will hurt you. Let her know that you found out what she's been doing, she's going to probably denies it, then move on. But make sure the info that you get is 99% true, not some bad info from someone who may not like her.