What Causes Constant Pain In Left Knee?

Hi, I have recently had an extremely painful episode with my left knee, over a number of days it became very swollen, very painful with difficulty straightening it, getting upstairs and walking. This has happened before but it was mild compared to this and was not as bad. My right knee has slight pain sometimes but my right foot & ankle swells most days. I drink a lot of water, usually approx a litre daily. I am 5ft 3ins in height and standard weight for my height and frame. I do about half hour walk and half hour (approx) cycle daily, swimming once a week. I seem to struggle with a lot of fatigue some days. Im usually in bed 10.30 - 11.00 and get up about 8.00 - 8.30. I have started using epsom salts in a bath twice a week and there has been a slight improvement. I also got a standard painkiller & anti inflammatory tablet in a chemist. I am careful of what I eat and don t use extra salt in food or sugar. How can I prevent this recurring? What can I do to reduce swelling in my right foot? Thank you, Fiona (aged 58)