Hmmm I remember a cat whos base of tail and bottom of back was really raw and bloody because of fleas but you say thats sorted now.

i think it sounds like what vet said, bum glands. And if she has had irritation on the skin too cos of the fleas in the past, her skin and underneath is probably very sensitive now.

Trip to vet definitely and maybe start introducing some wet food, as dry food has fibre but it also dehydrates and she may be consipated if she doesn't drink enough water to make up for all the dry food. I give mine wet and dry.

keep us updated.