Should I be upset at my mom? Is she being selfish or am I?

Well it seems to me that you have every right to be upset with your mother. Some advice, no one not even your mother can do anything to you that you don't allow or make you do anything you don't want to. First, maybe you felt obligated because you live in your mother's home and didn't want to stir up anything that will cause your living situation to be strained or cause you to have to get out right at this moment. That is unfortunately a by product of living in someone else home. Second, you are allowed to have plans and enjoy yourself in those plans. Next time when you have plans turn off your phones until you have ended your evening. You are an adult so have a conversation with your mother and set some boundaries that you both agree on and stick to them. Good Luck!