My horse keeps on acting up? why?

It could be the medicine, and he could be having a bad reaction to the medicine. He could also be pretty sick, or maybe hurt himself. Maybe that back sore could be what's causing the problem, but I wouldn't know how long he's had it. It could be that the rubbing could be making it worse (like itching a mosquito bite) and when you put the saddle on it could just hurt him so much he wants you off, no matter how much he loves you.