Yesterday my Rottie/German Shepard Dog (2 yrs old) started placing her tail tight against her bottom and never lifting it at all. If you try touching her tail she attacks your hand. She also has problems sitting down. She is contently licking the bottom part of her tail. It takes her several tries and ends up sitting on her hip till she can get down on the floor. She also is walking around with her head drooped. This morning she was laying down and every few seconds she would get a small "tremor". Should I call the vet or see if she comes out of it? :(

I think you should go ahead call the vet. She is definitely distressed. It may or be something that will go away in a few days. Putting off risks a turn for the worst over the weekend when you will pay through the nose if you find a vet.

Yes, please call the vet. Initially, while I was reading your description of what she was doing I thought she needed an anal gland expression. But, her behavior isn't atypical and there may be something else going on. As labman states, please don't wait. It is always better to be safe than sorry. Please let us know how she is doing.