Sudden Weakness in Right Knee with Spastic Diplegia CP

Been having a very interesting situation and hoping to see if anyone else has experienced this... I have Spastic Diplegia CP that mainly affects my legs with the left shorter than the right by half an inch. At random times rather suddenly and with no explanation, I'll experience severe weakness (with little to no pain) in my right knee where for about 3 to 5 days on average, I can barely put weight down on that leg and have to rely on a cane to get around and hold myself up...not to mention more swelling than usual and what I think is fluid in there. By the 6th day, everything goes back to normal like nothing ever happened just as suddenly. X-Rays have found nothing wrong repeatedly and my PCP and pain care doctor are at a loss as to what this is.