Should I be mad at my girlfriend and what should I do?

So I (22m) and my girlfriend (22f) are both in college and have been in a relationship for a little over a year and last night I drove her to a party that a few of her friends were going to. While she was there one of them suggested they go out to eat the next day and she agrees. The next day she calls me and lets me know she’s going out to eat with her friends. The issue I have with this is that we had made plans a week prior for her to come to my house today and I had already bought food for us to cook together and now she wanted to cancel just a few hours before she was supposed to come over. We got into an argument and she admitted that she was wrong for not asking me before she agreed but that she was still going, which kind of made the apology not seem sincere because how can you admit your wrong but still do that thing. For clarification these are not friends she doesn’t see a lot, but they are newer friends and she said she wanted to go because she felt like she was “being invited to do things with the cool kids” which to me sounded like something someone would say if they were in high school, like we are in senior year of college and you stood up your boyfriend who had already bought you food and made plans well in advance for some drunkenly made plans with some cool kids? Personally I found it incredibly rude and disrespectful and I don’t really know what to do.
Should I be mad at my girlfriend and what should I do?
Post Opinion