My grandma has metastatic breast cancer in her ribs & spine, she keeps getting these hot flushes. causes?

Medications?: Ask her medical oncologist. Some of the treatments she is getting could cause those symptoms. Is she having fevers? The medical oncoligist would likely want her to watch for signs of fever, infections, or check for low white blood cell count (neutropenia).

Hormonal therapy?: If her breast cancer type was sensitive to estrogen and/or Progesterone the her oncologist likely is treating with antihormonal therapy. This is in the form of a pill taken daily. By blocking the receptors or creation of these hormones you effectively cause symptoms similar to what occurs during menopause, when the ovaries greatly decrease their production of estrogen and progesterones.