<human>: Provide a brief overview of the text.

Alright, so, I could really use some advice.  
I've been casually dating this guy for almost 3 months, he's 27 and I'm a 25 year old female.  We never defined the relationship, hence "casually dating".  I preferred it this way, it was much more chill and we were getting along great, until yesterday.

Last night, a couple of our friends were throwing a costume party.  I had been invited by another friend, so my guy friend was not aware that I would be there.  Earlier in the day, he told me he was just going to chill at home that night.
Well, when I showed up for the carpool to the party, he seemed surprised and didn't really want to talk to me at all.  I just figured something else must be bothering him.  

At the party, he avoided me all night.  I was kind of hurt at that point, but I brushed it off.  Later on, I saw him talking to another girl.  It didn't bother me at first, but he ended up talking to her all night.  He was supposed to split a cab with me, but I ended up getting a ride home with a friend.  

He had an after-party at his house, which a friend of mine attended.  The girl he had been talking to all night was there and he saw them make out and go upstairs together.  

We weren't really in a relationship, so I don't feel like my hurt feelings are justified.  If I were to bring it up, I think I would just make a fool out of myself.  I've come to reddit to get some unbiased advice from someone who is not closely involved.

What do I do in this situation?  Should I just stop seeing this guy?  Do I even have a legitimate reason to be upset?
<bot>: Been casually dating guy [27m] who hooked up with another girl at a party I was at, don't know how I should feel about it or what to do.