Ok, so she is doing it now if I understand you correctly.
There is problem with the nerve signals going to her foot. The nerve signal originates in her brain and travels down her spinal cord. Since she is acting otherwise normal, I do not believe her brain is involved. It is common for older dogs to have degenerative changes in the vertebrae of the spinal column. One twist, or misplaced jump, and the vertebrae can become unstable putting abnormal pressure on the spinal cord and the nerves going to her hind legs. The spongy material between her bony spinal vertebrae, are called "discs." The discs can become damaged, and wear out, as well. This when the disc material goes out of its normal position, this puts abnormal pressure on the nerves, and spinal cord, as well. This is known as a "bulging disc" or "slipped disc" or "herniated disc."
It can sometimes be diagnosed with plain x-rays. Sometimes an MRI is needed. Often, the MRI is too expensive for the owners so we just treat the disc, or nerve swelling, with high short term doses of steroids a long with rest.
If she has arthritis in her hips, that could affect the nerves to her hind legs as well.
I am pretty sure the problem is in her back though.
The only other thing it could be is a blood clot, in a major vessel, that supplies that leg. Check the temperature of that foot. If that foot feels very cold, there is a problem with the blood flow and she needs to go to the emergency vet right now.
Otherwise, I would try to restrict her activity tonight. Take her to her regular vet first thing in the morning.
I hope this helps.