Ask her doctor to do a few tests- if not done already
1) Full Blood Examination and Blood Film- FBE - To look for signs of infections with rise in white cells, and if so the subtypes of white cells.
2) Thyroid Function Test- TFT - To see if she has hyperthyroid disease. Feeling hot at winter, feeling hot at night, shaking - could be a thyroid disease.
3) Liver Function Test- LFT and Creatininine Kinase- CK--> She's on lipitor, and she had problems with walking, and now this drug fever. Lipitor and other statins have an idiosyncratic time with onset of myositis and other inflammation of the muscles. Fever could uncommonly with one of the symptoms.
4) Also urine test- as old people can often get asymptomatic urine infection, with only fever being the only sign.
Obviously if fever is persistent, then other things such as CXR.
If the basic tests are all normal though, i'm gonna be
Han Solo