Lately I've had tightness and some dull pain behind my left knee. The symptoms worsen throughout the day; first thing in the morning it's fine though.?

Knee pain on standin. Pain in the back of your knee that worsens as the day goes by and resolves overnight is typically due to a bakers cyst. This is a fluid filled sac that forms in the popliteal fossa, which is a space behind your knee joint. It will cause you pain and a felling of fullness, but should not cause leg, knee or calf swelling, redness or heat. Any of those symptom need evaluated by your doctor.