What Causes Pain In Left Knee?

Hi, i have had a pain in my knee for 1 week now, its my left knee, painful all the time but worse upon resting. Pain feels like a bad cramp/ache pain, with a sharp pain when i move joint. Pain is situated mainly to right side of knee (where both knees meet) and above kneecap. Have been taking brufen and CoDydramol with no effect. Im waking 3-4 times a night and having to walk round the house to ease the pain.What do you think this could be? any help would be appreciated.