You mother is having rheumatoid arthritis, since she already got a hip replacement due to hip joint damages.
People with rheumatoid arthritis are at increased risk for developing fibromyalgia, which is a syndrome (set of signs and symptoms) that may be associated with different diseases.
Although RA is most commonly associated with joints of the hands and wrists, it can also affect larger joints, such as the hips, knees, and shoulders.
Hip pain, with discomfort and stiffness in the thigh, lower back, and groin, are symptoms of hip joint being affected by RA.
Arthritic pain is known to be one of the most intensive pain in clinic, that seriously affects patient's life.