I am only 21 and have been feeling this pain in my knee when I kneel on it. If I kneel on my left knee I get a sharp pain like my knee is kneeling on a pin that is digging into my knee. It is a sharp enough pain for me to get right back up off it and switch to my other knee. I work in a convenience store and have to kneel down to get into certain places so I feel this pain every time I kneel on my left knee. The pain is the bottom of my knee cap on the left side of my left knee. I did karate for years and I do not know if it is an injury I didn't know I had that is just acting up now.