If the scabs are mainly along the back, and are quite red and lumpy then it sounds very much like a condition called FAD (Flea Allergic Dermatitis). This is often called milliary dermatitis.
This is due to an allergic reaction to the bite of the flea, not necessarily due to a heavy flea infestation. Therefore only a small amounts of bites are required to trigger this condition.
I suggest you get her checked by a vet who (if confirms this diagnosis) will advise treatment with a prescription flea treatment (such as Advantage, or Frontline) and may also give a short acting steroid injection to combat the allergy.
Depending on the severity they may also give you a household spray.
If it is FAD then you must keep up to date with good flea control and treat all pets the same, not just the affected on.
There are other things it could be, so get her to a professional who will be able to examine her and give you a much better idea of what is going on. Good luck.