I just switched from a normal cast to an air cast and my foot hurts more and is numb, what should I do?

Swollen. It's probably swollen, but tell your surgeon what happened. You may need to get out of the aircast and into something more stable. Are you allowed to bear weight on it? If so and if you have been, stop and rest and ice and elevate your limb.
Tell. Your doctor. Numbness could be from tightness of a bandage or immobilization device. You may not be ready for air cast yet.
Elevate & deflate. Elevate that leg and let a little of the air out / pressure off and see if the numb feeling improves within 20-30 minutes. If not, call your orthopedist to describe your problem, as it could be an indication of a serious problem. If cannot get a hold of your dr after an hour or two at most, then go to urgent care / er to get it re-checked.