this has been happening to me for a few years, i can't squat down or bend my knees to far and sometimes my knee goes from under me, they think i have psoriatic arthritis due to 35 years of psoriasis but i know i do have arthritisin in my knees, hips and ankles, some of my family got arthritis in there 30s one who was a footballer, if i fall i have to shuffle on my bottom to some steady furniture to pull myself up sometimes ibuprofen can help i can't take it due to cancer so ask you doctor if you can and try tablets or voltarol cream, i do lots of walking and was told by an elderly gent a few years ago that i had a purposeful walk as i walk quickly, i am 58 and miss not being able to wear my court shoes, now it is tainers and velcro flats,,,,lol