I fell over quite a few years ago one afternoon, banged my knee & ended up with fluid on the knee! I went to bed the evening after I had banged my knee & when I woke up the following morning couldn't bend the knee that I had banged. I went to casualty & at first they were going to drain the fluid off the knee, but eventually decided to try a course of 'heat treatment' on my knee. I went to the hospital about 3-4 times, lay on a bed for a half an hour while a heat lamp was directed on my raised knee-they put a cushion under my knee. My knee was soon back to normal & I have had no problems with my knee since. I would agree with the others that your first course of action should be to make an appointment with your GP, and they will be able to properly diagnose & recommend a course of treatment for your injured knee. Peace & Love.