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Unfortunately, this is only something that can be ascertained via an xray. The tail is an extention of the spine and contains bones, discs and vertibrae. Any spinal injury can bring on ths symptoms. Even impacted anal gands can be a cause. If she is not spayed a uterine infection is also possible.
Try applying very warm compresses or a heating pad (on warm) to the back and tail. I am hoping she will improve but if she does not you will have to have her seen by a vet.
Please let me know how she is. If you still have questions I will be happy to help.