There are a few possible causes for a dog to lick their butt. The most common reason is impacted or overly full anal glands.
Anal glands are scent glands that are located at 4 o'clock and 8 o'clock around the anus. Normally a dog will express a few drops of smelly, brown liquid at the end of a bowel movement. But in some dogs, particularly small breeds or overweight dogs, the anal glands don't empty like they should. Then the fluid builds up in the gland and becomes uncomfortable to the dog. Some dogs will scoot their butt on the ground try to express the glands, some dogs will lick.
If the glands get too full, a bacterial infection can start and cause an abscess which will eventually drain blood, pus, and anal fluid on your floors. : (
Some dogs with allergies will also lick their butt frequently, especially with a food sensitivity. But they will often lick other places on their body, like their paws. So if Ehm is only licking her butt, it is likely due to anal glands. Tapeworms can also be somewhat itchy--you might notice a flat white worm on her stool or something that looks like a piece of rice on the fur around her tail.
In either case, it would be good to take Ehm to your vet and have her anal glands checked and expressed before they get infected.