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Hi there and thanks for your question.
What you have described is commonly called by vets "miliary dermatitis". The cause of miliary dermatitis is almost always an allergy. By far, the most common type of allergy to cause this is a flea allergy. Some cats are so allergic to fleas that all they need is one bite to set them off. I would recommend a good prescription product for fleas such as Advantage or Revolution. Unfortunately most of the over the counter products for fleas just don't work well.
If this is an allergy to something else, a cortisone shot given by your vet will likely help. If it only helps temporarily then you may need to discuss changing foods with your vet.
The other possibility is that there is a wound present and it is getting infected. This is less likely though. You could certainly try a small amount of neosporin/polysporin 3x daily, but most likely she will need to see the vet.
I hope this information helps! If you have more questions regarding this problem, just hit reply. If I am online I will respond right away. Otherwise I will respond as soon as I come online again. Or, if my answer is sufficient, then please hit accept. Thanks!