My Right Knee Keeps Fluid On It. A Lot Of

my right knee keeps fluid on it. A lot of fluid. I have Ankylosing Spondylitis. My knee feels like a hot knife is being stabbed in it and twisted. I can hardly bend it due to the swelling presumably. I can bend it back and forth like normal just with little movement. but the right to left movement is very painful. I've had the fluid drained off numerous times and had steroid shots in it. all to no avail. Very painful and its on fire. X-rays came back negative with "no structural damage."