I had a similar thing with one of my cats, bald tummy but he did not over groom scabby patches and bald back legs. The vets tried the defleaing 1st with no joy. They also sugested it could be a change in food or food ingredients if you haven't changed that. I could also be enviromental, have you washed the bed in a new washing powder recently. My cat was going to be on permenant steroids to control it as we could not get to the bottom of it and he could manage on what the vet assured me was a very low dose for a cat.

The good news is my cat seems to be responding well to changing to a raw diet, this is all very new at the moment though so time will tell. Hope you find the cause for your puss soon though as I imagine it must be uncomfortable.