My cat has been scratching excessively for a couple of weeks now and I'm noticing more frequent grooming.?

I've taken her to the vet....no fleas, ticks, mites. etc. She is strictly an indoor cat. I was given an antihistamine for her, which does not help. I was told about the possibility of a food allergy so I changed her diet to one with "limited ingredients" which are lacking the usual food allergy triggers. That's not helping either. I tried cortisone spray for cats which does nothing either. I tried vitamin supplements which contain Omega oils to see if that would help if the problem is just dry skin or dandruff. She has no redness on her skin and isn't getting any bald patches. She is acting normally. But, she seems uncomfortable, scratching every chance she gets. Any other suggestions?