uhh.. agressive??

So fred was neutered yesterday and today he is being SO agressive. He is NOT usually like this but I m a little nervous. Maybe its the medicine?

I went to take a bone from him and he made this god-awful bark and his whole body was shaking....

what do i do?

He's probably in pain and reacting to it... I'd let him be for a little while. He doesn't know what's going on right now