I am sorry to hear that your girl is in pain.
Perianal fistulas (or often referred to as anal furunculosis) are tunnel-like lesions that form between the skin and the deeper tissues in the area surrounding the anus. The condition is usually very painful and can cause a lot of discomfort. The exact cause is unknown, however it is unfortunately very common in German shepherd dogs.
It used to be thought that a contributing factor was their broad based tail and the way it is carried. Nowadays it is suspected that the cause is immune-mediated. This is a term to describe when the immune system overreacts and starts to attack itself. The disease responds to various immunosuppressive drugs- and quite often systemic (tablets) and topical medication will be prescribed. Supportive therapy with antibiotics may be required to take care of any secondary bacterial infections that may occur.
A dietary trial may be suggested if the dog has colitis (inflammation of the colon) at the same time. Surgical treatment may sometimes be advised of which the main aim is to remove any diseased tissue. I would suggest speaking with your veterinarian about the most appropriate treatment for your girl.
I hope she is feeling better soon.