What Causes Knee Patella Pain On Straightening?

I have pain just above the patella on my right knee. I can run and physically strenuous activity on it with no problem but when I sleep it is very painful to straighten my knee back out. I am 30yrs old and moderately active. I have done some knee exercise which has helped through the day. If I want to straighten my knee at night with no pain I can put pressure on the inside of my patella and push outward. any ideas? This has been going on for over a month.