I have experienced pain, stiffness and swelling in my right knee. It hurts going up stairs. Is especially stiff when I sit for just as little as 20-30 minutes, It feels weak and it is not going away. I cannot lay it flat for any length of time. I try work out several times a week and it has affected the quality of the work out and even walking for any length of time
There are many conditions causing your symptoms. Could be rheumatoid arthritis, osteoarthritis of knee(old age), due to trauma, if there is fever then it could be infection, rarely can be tumor.