Cat Hair Loss from Anti-Nausea medicine

I took my cat to the vet two weeks ago because he wasn't eating. They gave him a shot that they SAID WAS ONLY nutrients and fluids. I called today asking what they put in, and it turned out they put anti-nausea medicine in his shot as well. 

The place where they gave him the shot is still scabby and where the fluid was put/fell, he as lost all his hair. At first I thought it was allergies to food, but no. It must be from the shot. Why else would he lose hair EXACTLY where they gave him the shot?

I've changed up his food just to be sure it wasn't allergies, but has anyone else had a similar experience where a shot at the vet caused hair loss?

How large a spot are you talking about here?  And it got scabby?  I've never heard of that happening from an injection either.  Normally you can't even tell where an injjecton was given.  Was she scratching at the site or something?  WHERE is it located on her body?