I'm really upset my girlfriend had plans with me and then ended up changing them.
Why are you so upset about her making different plans?
It makes me feel unimportant.
I understand why you're upset but you shouldn't get angry before discussing the issue with your partner. Being quick to judgement will cause your partner not to trust you.
You're clueless, that's why. You don't understand how important my feelings are.
I'm sympathetic toward your situation, but getting angry right off the bat isn't the way to go about approaching the situation. You should talk with her girlfriend and let her know that doing things like this upset you. Most likely it would prevent her from doing it again in the future.