Since she is holding her tail down, she could have overly full anal glands that are causing pain around the tail. Dogs have two glands that are located at 10 and 2 (like a clock) beneath the tail. These glands normally secrete a smelly substance every time the dog poops. Sometimes, these glands get overly full or infected. If this is the case, the vet can express the glands and give the dog some relief.
Until you take care of this, he might lick alot and he might even drag his bottom on the ground.
No matter what, you need to see the vet with him so that you can get to the bottom of this tomorrow.
Many dogs will hold their head down when they are in pain so go ahead an see the vet first thing tomorrow.
Let me know if you still have questions.