Cat skin problems?

[Okami]

Hello, I was wondering if anyone here could identify if my cat has a skin issue.

Background: I was away for 11 days and i left my cat at home with a cat sitter. During this time, he was fed the same food but was only fed once a day (1 full can), and the cat sitter didn't notice any abnormal behavior, or lethargy in my cat. When I got home I didn't notice anything usual about my cats behavior, I did notice that there were 3 odd pimple(pin prick) sized bumps on his head and neck, a small scabbed bump, and a medium sized red scab on his belly (picture below).

I took him to the vet, and they said it's not an allergy, because his diet hasn't changed, and if it was an allergy the outbreak would be more severe rather than 4 small bumps. It's more likely he got stuck somewhere he wasn't supposed to and roughed himself up a little when he was trying to get out.

Also The vet picked at the scab and bumps, and my cat showed no reaction at all.

Has anyone ever had this happen before?
Thanks guys!