Black/Brown spots, itching, on Chihuahua?

My 3 1/2 year old white w/fawn spots Chihuahua (6lbs) over the course of the summer, has developed a dirty look to her underbelly. Then came small black/brown spots, uneven ones that would appear in clusters or randomly on her belly and under legs- these spots seem to be adhered to the skin, but if you scratch your nail across them a bit, they peel off, and after a day or two of being there, they rub off easily. The rest of the skin had this dirty-but-not-really appearance. She could have just be thoroughly bathed and it would still look like that. She itched a lot with this issue. She has been to the vet three times (2 different vets). First one put her on allergy meds- didn't do anything. Second one put her on pills and doggie hydro cortisone- didn't do anything either. Finally got her on a holistic herbal fungal/skin infection treatment (liquid and pills), and it has cleared up her belly! But the condition moved to her back end, where we hadn't been applying the liquid (we are doing so now). My question is: does anyone know what in the world she has??? The holistic girl's first idea was yeast, but she isn't so sure now, since it cleared up so quickly, and with yeast the spots apparently aren't supposed to "rub off." She doesn't have fleas either. Any insight would be greatly appreciated!