Foot Pain. I Have Severe Osteoarthritis Affecting Mostly Hips And

Foot pain. I have severe osteoarthritis affecting mostly hips and knees (already had 1knee replaced). Back of my right foot has had a large lump on it that hurt for years. Haven't been able to wear shoes with backs for years. Now, back of left heel about even with ankle bones is extremely painful and swollen. Unable to bend foot carefully or walk with any ease. I wear good shoes, either Birkenstock or Alegria. Any suggestions?