knee problem

Hello, I hope someone can help!!!! My left knee has been bothering me for almost 2 months! I never injured it other than a lot of walking. I'm 100 lbs overweight so I do know I need to lose weight A.S.A.P! however I do think there is another problem causing this problem to linger. After being on my knee more than an hour, there is swelling on the left side slightly above the knee cap, and severe pain sets in. I am forced to get off my feet for a least 30 minutes and then the pain subsides until I put weight on it too long again. Looking forward to a great suggestion. Sincerly, Lo O