She just got out of a bad relationship a few months ago and we have been taking it slow because she was confused about her feelings
That's all the warning you needed to realize you had to protect your heart, and maybe not give it to a stranger, or have high expectations for a great, perfect future. No she didn't cheat because you were going slow enjoying getting to know each other and enjoying it, and I do understand the disappointment, but don't miss the honesty of it either.
How you handle it going forward is the main thing because the dilemma is how to cope with YOUR feelings and not the actions of a confused female fresh from a bad break up. It was your mistake to think you were building something big in the first place given the warning signs.
Reassess your expectations and see the part you played in this, and be as honest as she was and if you cannot cope with the reality you are in, remove yourself completely from the situation until you can.
Sorry guy, but you set yourself up as a rebound to a hurting, confused, and very vulnerable female looking for an escape from the bad relationship. Had you recognized it and kept a safe emotional distance with a new text/phone buddy/friend/acquaintance, you would be disappointed, but not surprised by her actions.
That's the problem the shock and surprise she ruined what YOU had in mind. It was probably closure for her. Even going slow doesn't prepare us for the actions of one that is still carrying baggage from the past, and barely into the healing process.
You really should have know she needed a friend, and not romance. Now you know that for sure.