This could be due to compression of the nerves in the lower spine . Needs evaluation.

Plz get mri of lower back that's lumbosacral spine and x rays of both knee ap and lateral views . She is most probably suffered from neuroclaudication due nerve compression in that back . This leads to feeling of heaviness , numbness , pain dragging sensation, weakness in lower limbs specially calf and foot . Most during sleeping , prolonged standing and walking . Bevause spine is straight in all three positions .she must not be able to stand for prolonged period as well as walking distance could be difficult for her . Hope this answers ur questions.