Most probably your grand mother is suffering from arthritis
of knee, gout or back problem may be there. Take x-ray of b/l knee and back (lumbar region) of anterioposterior & lateral view and a blood test of uric acid. After that consult with your nearest physiotherapist
or orthopedic Dr. Take physiotherapy
treatment for knee pain
paraffin wax bath, tens, hot pack for next 10 days regularly. Followed by quads. & hams. Strengthening exercises.