Suggest Treatment For Stiffness In Knee After Strenuous Job

I am a 39 year old male and I have a physically demanding job. I do not recall any recent injuries to my knee. Recently I have been experiencing pain to the back of my right knee. The pain is localised behind the knee; however when I stand after kneeling I feel slight pressure to the front behind the knee cap. After physical activity when sitting down for a few minutes my knee stiffens up to a point that I can hardly extend my knee and put much weight on it; however, once I start moving again the pain subsides in couple of minutes. It never feels 100% anymore. I notice in the mornings it takes a few minutes to get myself moving and the condition seems to be coming on more often. I am also noticing my left ankle is becoming stiff as well. Not sure if it is because I am compensating. Am I just getting old with Arthritis or could this be the start of something else?