Do you mean her tail?  The long floppy thing that wags or her actual bottom?

If it's her actual bottom, I would be worried about anal gland problems.  The anal glands are located at 10 and 2, like the clock, below the dog's tail.  They normally empty themselves but can sometimes need intervention.  They can get infected and need to be drained and the dog might need antibiotics.  In severe cases, the vet might even remove the glands.

I suggest that you take a look at her bottom and see what's there.  You may just need a trip to the vet if the glands are impacted or infected.

Let me know if you have more questions.

Hello,

You might also want to rule out fleas, ticks or mites on your dog. Very often they will burrow into the skin and cause much discomfort to your pet. If he is trying to bite at the tail, this may be the problem as well. Being that they can't tell you in direct language, the need for constant petting may be his way of hoping you can "scratch the itch" for him. If you can't tell yourself, you should have this ruled out by your vet, however, they do sell over the counter flea and tick medicine- or it can be prescribed as well.

Hope this might help. Take care.