I Had An Incident While Just Walking And My Knee

i had an incident while just walking and my knee kind of locked up. had xrays and mri and knee is structurally sound, dr said must be bruised bone. i had done nothing to cause this......no falling etc. at first pain focused behind knee and since pain focused inside area of knee. then I had a 5 day unexplainable fever and put on antibbiotic. since off antibiotic lef swells from knee to foot. i am concerned why that would be. any ideas?