I'm having extreme anxiety about going to a concert in very hot heat, what if I throw up or pass out?

I'm thinking. That you are going with people who care about you and who would take care of you if something untoward happened and that this concert is coming up soon. Not sure if you have time to see a psychiatrist for help before you go. If you do have time, make an appointment. We are here to help.
Stay hydrated. If you are just prone to pass out in hot environment, you need to dress in cool, loose-fitting clothing, stay hydrated with water, gatorade, etc and avoid alcohol. But if your problem is actually an agoraphobia ( extreme fear of public place), you probably will need a professional help.