You didn't do anything wrong. Nothing. Nada. Zilch. Zip. You are being "excused out" meaning whatever lame-ass excuse that can be drummed up for bouncing you out will be given. Your medical condition had nothing to do with it or your class prep. This is extremely common everywhere in China and has been going on forever to foreign teachers. Move on and don't defend yourself, don't blame yourself and don't listen to the B.S.