Knee pain

I get a lot of pressure built up in my knee if I keep it flexed, I have to keep extending my knee to pop it and relieve the pain

[Michael L Gross, MD]

Your description is a bit non-specific, but, pain after prolonged sitting often reflects patella-femoral problems. This used to be called a positive "movie sign", because at the movies, you had to sit on the aisle so you could sit with your legs extended.
This usually responds to NSAIDs and quad strengthening  exercises.