Sores on Sassy cat glutin, protein or inhalant?

Sassy, one of our rescues had sores all over her back which looked like mosquito bites. They apparently iched and were sore. They are about the size of a Pencil eraser, red but no draining.

So off to the vet where she was given a cortisone shot. Sure enough in about 3-4 weeks they dried up.

The guy where we rescued her was feeding a cheap kibble. We switched her to ground chicken supplemented with blue buffalo/before grain mix.

In about 2 weeks the sores began to return. She was brought inside as soon as we captured her.

She was put on before grain for 10 days with no change in the sores.

So we have ruled out mosquito's.

The vet says most cat allergys are to proteins, not Glutin... Truth?

The vet says might be air born such as molds, etc. another cortisone shot and frost they should "clam down"

Questions and observations

Could it still be glutin even if we see no change on grain free for ten days?

Could it be protein (chicken) and should we change to lamb/rice?

If it were airborne would we not have noticed a difference from the old barn area to inside our basement?

Any suggestions for a cause... Treatment?

I hate to keep giving the poor thing cortisone but what should I do?

Oh also $70 a pop!

Have you had a culture done to rule out any type of fungus or anything like that?  It could be a chicken allergy.  You might try a limited ingredient diet or a simple protein like duck.  I'd try to stay away from rice as that's also a grain.