It is impossible for anyone here to diagnose what might be wrong with your dog from the information you have given us. Hair loss can be caused by many, many things, from something as easily curable as flea bite dermatitis to something as serious as Cushing's Disease. Even if you were to give us more info, for something like this you need to make an appointment with your vet and let him or her examine your dog. That's the only way to get an accurate diagnosis. Please do not wait to call. If that is a cyst, and, for example, it breaks open from her scratching it, your dog could end up with a terrible infection.
Please keep us posted and let us know what the vet says.
What Ghilly said is so true.
My dog once had a little cyst on her back. In itself, it was harmless. But one day when I stroked her I noticed she winced. I took a look, and a nasty abscess had developed on top of the cyst. I took her to the vet and had it drained and treated professionally. She was fine after that, and since.
The vet told me this can often happen if a cyst gets scratched or damaged.
Ginger and ghilly are both right...its best to let your veterinarian know about this and notate it in the medical record, even if it is nothing severe.
You might put together a little more detail (breed, age, last vet visit,etc) and post your question at the Ask the Veterinary Expert forum here at MedHelp. There is even a forum just for skin issues in pets that is monitored by a veterinary dermatologist.
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