Why is my cat grooming herself excessively to the point of baldness?

Hello everyone this has been going on for a while, but my cat has been excessively grooming herself for a few months. At this point her inner legs and stomach are near bald. When we brought to the vet they just gave her a cortisone shot. It wasn't very effective, it lasted maybe a week but then she started up... show more Hello everyone this has been going on for a while, but my cat has been excessively grooming herself for a few months. At this point her inner legs and stomach are near bald.

When we brought to the vet they just gave her a cortisone shot. It wasn't very effective, it lasted maybe a week but then she started up again.

Any ideas to what is causing it?
Update: edit: she is an indoor cat with another cat in the same house, they have been together for multiple years.
Update 2: Edit 2: Already brought to the vet and had blood work done, no mites or fleas either. So it's either a food allergy or stress than. Not sure what could cause stress since though.
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