Didier,
What you describe could be caused by many things among them, platinum based chemo, radiation affecting nerves through the neck, less mobility because of treatment and fatigue, dehydration, buildup of lactic acid from the liver trying to detox the effects of the chemo, poor circulation from the chemo/radiation/less mobility, thryoid, etc. Lack of excercise will cause muscles to deteriorate and cramp.
Keep your mother hydrated and check her blood work regularly until it is all normal. At a minimum, she should have blood work to include CBC, hepatic panel, TSH, electrolytes done at least once per month. She could also be taking calcium, B6 and B12 at doctor recommended dosages. The body is pretty beat up right now and normal nutrition alone probably wouldn't satisfy the body's need. I am sure she is not eating what she was before treatment which compounds the nutrition side a bit more. Have the doctors check the blood stuff first and then recommend a good healing regimen of vitamins.
Ed