Hi, how old are you? Any family history of arthritis? Usually multiple joints are involved in rheumatoid arthritis, it is autoimmune disorder. Investigations like ESR, RA factor and X ray of relevant joint is undertaken.
Other possibility is gout, but in gout usually single joint is involved. Please consult orthopedic surgeon.
Perhaps you suffer from fat pad inflammation which located behind the knee cap bone,It can referral pain to joints in feets.