I have a tight rubber band feeling and pain in the back of my knee when standing up from sitting or my leg/knee bent for a long particular time?

Stifness: If you sitting for long it is normal to feel little of "knee stiffness " . Also if you do not stretch on regular bases your back tight muslce tendons ( " hamstrings" ) become very tight and it can cause discomfort when gets up after sitting. Keep your body weight down to BMI <25. Extra weight multiply for knees x 4. Regular exercise.If minor discomfort put ICE and take Ibuprofen 400-800 mg.