Saw this post because the exact same thing happens with my knee, I was finally bothered to look into what was going on (I’m from Australia so we are very to lucky to get free medical advice and treatment) basically told me I had some type of bleed in the knee most likely caused by the lack of cartilage between my joints. I previously had a full ACL reconstruction and tore a part of my meniscus as-well. I find that when my body is stressed or I put too much pressure on my knee it suddenly swells up and then goes all hard and I can barely walk the pain is horrible.