It's worth checking her thyroid function:
"Some studies have related hypothyroidism to muscle dysfunction... Furthermore, studies ... demonstrated that thyroid hormone action on skeletal muscles affects mainly type-I muscle fibers, which promote slow contractions and are most prevalent in the postural muscles recruited during prolonged effort" [1]
Also, not sure where you are in the world but your doctor telling you your thyroid hormone levels are ok unfortunately may not necessarily mean they are ok... There is a controversy about reference ranges amongst experts [2], and I think it's wise to pay attention to that.