Sounds like a flea allergy - it's the most comon cause of the skin condition, called miliary dermatitis. The spots will be mostly on her back, especially towards the tail, and she may be looking slightly bald there as a result of over-zealous grooming.
The trick is to control the fleas - even one bite can trigger the allergic reaction that causes all the spots. A good flea treatment is needed every four weeks, and you need to treat the home too - 95% of a flea population lives as eggs, larvae and pupae in the carpets and flooring of the house. Wash everything that can be washed and vacuum everywhere the cat has been, then spray the areas involved with a good insecticide (e.g Acclaim). Hopefully you'll be able to get on top of them, they can be hard to control.
Don't worry if you can't find any evidence of fleas on your cat - often they groom themselves very thoroughly to remove the source of the irritation and you won't necessarily see any adults or flea dirt. If the flea treatment worked before, the fleas are the likely cause. Let us know how you get on!
(I'm a UK vet).