Hello, welcome to JustAnswer! I am a Veterinarian and will help you right away!
can be a sign of itching.
Itching in the area right above the tail head I would consider a few things:
1. fleas - this might be the easy rule out. Your dog might not have fleas. BUT, typically they tend to congregate right around the tail head along the back. You might see flea dirt accumulate there which is a black kind of dirt. When you add water it turns kind of red. It is really digested blood.
2. anal gland irritation/perianal itch - anal glands are scent glands found in the dog. They are about 1.5 cm inside the rectum. They mark territories, we think. A dog with a very well defined one is the hyena. It's anal glands can scent mark that lasts up to 1 month. Cats have them too, a cat that has a really strong anal gland is the skunk.
BUT, in the dog they can become full and need expressed by a Vet. This becomes itchy and irritating as they fill up.
Perianal itch can be related to food allergies, diarrhea, constipation. The pet tends to lick and chew at the itchy rear.
3. orthopedic pain - in this breed you can never rule out orthopedic pain. It doesn't always have to be in the hips. It can be in the lower back or even at the tail as well. In the lab I have seen a condition in which the tail is placed slightly higher than the rest of the spine at that location. It creates instability and pain. I think that this might be related to the cause of the condition "limber tail" that you may have heard of in labs
So, that gives you a few ideas. There is not a specific cause I can give you without seeing your dog. BUT, you should discuss these with your Vet. Be sure there are no fleas, you might want to treat for them anyway if these are a problem in your area. Also check the anal glands, and check for pain. They might not be obvious to you. Your Vet will know where to push and pull to discern that.
Lastely, your GSD is a little young which is why I mention it last, but sometimes a base of the tail itch might lead to hair loss in that area. Hair loss at the tail head can also be a sign of hypothyroid disease. This is less likely in your young GSD.
Hope that helps,