Knee joint bears a major portion of the body weight.... It could be anything from an injury to a tear in ligament or meniscus to osteoarthritis.... Visit an orthopaedic consultant for further clinical evaluation.... Meanwhile apply cold pack for swelling to reduce....

If the pain is mainly in the knees and the small joints are spared it may be osteoarthritis which is age related. You have not mentioned her age. The pain gets aggravated with physical exertion like climbing up and down the stairs. Sitting on the floor and squatting etc. Ask her to avoid doing these activities. She will require x-rays for further evaluation. I would suggest that you first get here examined by a knee specialist and then take it further. Usually medication and physiotherapy can help relive the pain to quite an extent.

Your mother may be suffering from osteoarthriris of the knee. Get x rays done.