Cat with very serious skin problem?

My cat is 4 years old. A few months back we noticed she had very scabby, scaly skin. It looked like a sort of dandruff, but she had it all over her body and her fur was falling out in clumps. We tookher to the vets and he gave her pills but they didn't work. She has diarihha and often has red in her urine which is blood. She's lost loads of weight and now has big patches of raw red skin and no fur on these patches. Please, does anyone know what this could be? Have you had a cat with this problem?