This may be due to a number of causes, as she is taking rivista that means she has hypercholesterolemia and obviously as tenormin shows hypertension as well so both are leading causes of peripheral arterial diseases but that does not seems to be the case here as she has no calf pain while walking if I am correct. Or may be due to venous stasis like a DVT or varicose veins but again they are unlikely as it affects the whole foot and calf pain and varicose veins has veins visible.
Now is there any pain in the area is the area warm, if not this rules out infections like cellulitis, however it may be a fungal infection like a ring worm, but that doesn't seems like that as it has a ring with clear area in between. Also it is highly unlikely that pain killers are causing it, they will give a skin reaction known as erythema multiforme google it to see it pics it is not looking like it.
May be some autoimmune dissease difficult to tell which one. Best thing in this scenario would be to get a doppler ultrasound of leg done to rule out arterial or venous dissease. Show your mother to a good dermatologist or internal medicine specialist.
This is off topic but one thing I will advise you to do is tell her to stop taking large amounts of pain medications only take when required that means SOS, they have serious side effects. If its an osteoarthritis get it documented by doing a plain x-ray of knee, secondly get a DEXA scan done and if she has osteoporosis start her on bisphosphonates and vit. D and Calcium. Please refrain from using excessive amounts of pain killers, nearly all geriatric population in pakistan has osteoarthritis best treatment is weight reduction and exercise for strengthening the muscles.
