My girlfriend went on a date with someone else. What should I do?

Anonymous
I am on an exclusive relationship with my girlfriend of two years. The other day, she met a guy who had just transfer to our college. And chatted with him for a while (with me present) I acted cool without giving too much importance to it.

Later, we were working together on the same table doing homework. She left her computer open and I could see her calendar on the screen and I saw she had there an annotation about meeting this guy and having dinner together.

I did not say anything, but took note of the date and place. Then I went to the restaurant in the said time and I was able to see them having dinner together. She had, noticeably, put attention to wearing nice clothes and looking nice -nicer than she normally looks when she hangs out with me.-

I did not say anything and just left without being noticed after a couple of minutes. (I saw them through a window and I was protected behind neon lights and wearing a coat that covered me enough.)

From other sources I know this is not the first time they meet. I know they did arrange to meet a couple of other times before that in our college Campus.

What should I do now? Should I assume she is cheating on me? I really don't think its proper for her to be accepting invitations for dinner while being my girlfriend. It would be like me asking for girls phone numbers and asking them out. (Which I do not.) How should I handle this?
My girlfriend went on a date with someone else. What should I do?
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