Hi Diane, and thanks for your question.
While older cats are more prone to develop certain conditions that can cause pain and weakness in the hind legs, it's also possible that she sprained, strained, or otherwise injured either her hip or leg, when jumping down from a height, recently.
Because she's exhibiting this pain reaction, and is reluctant to walk, she needs to be evaluated by the vet, ASAP, and an x-ray may be needed to see if she has a hairline fracture of this rear left leg. Blood work will also be necessary to check her numbers, as older cats are more likely to get diabetes, which can affect nerve function in the hind legs (diabetic neuropathy), and also a cardiac condition called cardiomyopathy can cause a blood clot to form, blocking circulation to the hind legs.
Try bringing her to the nearest ER vet center, now, if you have one in your city. If not, look in your Yellow Pages, under veterinarians, for any listings including 'emergency hours'. The sooner you have her examined in person, the sooner you'll receive a diagnosis and she'll be treated to help her feel more comfortable.
I hope all will be well with your kitty.