How much does your dog weigh and what was the strength of the tablet?
I'm very sorry, when I read your question, the additional information wasn't there so I thought you were just asking about the pill consumption.
To answer the rest of your questions, it is unlikely that the xanax is still causing her to be like this. Given her age, breed, and what you are describing it sounds like she has some pain associated with either arthritis in her joints or neck or back pain. What I would suggest you do is keep her very quiet for the next day or so, no jumping, play etc and see how she does. If she is still acting like this then you will want to take her to your vet so that they can look her over and find out where and why she is so uncomfortable and then possibly do some xrays etc. They will also be able to give her medications that will make her more comfortable.
Again, sorry for that oversight. I hope she is feeling better soon.