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Is she on flea prevention? If so, what kind?
If you have a narrow tooth comb, I want you to brush her with it. Just brush the area where back meets her rump, right in front of her tail. Brush her over a piece of white paper and then shake the comb on the paper. Tell me if you see black flecks of dirt fall off.
You did get black flecks of sand?
If you did, she has fleas. Those black flecks are flea "dirt." It's actually flea poop. They consume the blood of the animal and then poop it out. If you wet one of the flecks, it will turn red and look like a drop of blood.
The GOOD thing is that your problem is easy to solve. Just swing by your vet's office on Monday and pick up some Frontline. It's about $10 and it should solve your problem. It will kill the fleas within 24 hours.