Why do I get pain in my left knee when it is bent for a while?

Alignment problem. Knee is not a perfect joint. Positional ligamental strain can cause it. You are too young to have it. Tone yourself by focused exercise.

Related Questions

Was weeding & kneeling for 15 minutes. When I stood up, can't bear weight on left knee w/out severe front knee pain. Doesn't hurt when sitting. Tips?

Conservative care . Could be an injury to the soft tissue structures within the knee, possibly the meniscus. I would treat with conservative care: rest, ice, and perhaps Aleve (naproxen) or similar product. Hopefully it will resolve in short order. Read more...

I have a cyst on my left wrist, left shoulder pain, left hip pain like a pinched sciatic, left knee pain, and left ankle pain. Coincidence? Left side?

Not all related. Hip snd lower psins can sll be related to a pinchedl5nerve root but theupper extremity stiff unrelated to lower stuff the srist cyst can cause shoulder psin through regonal paindisorder. Read more...
Left Side Pain. Have you had your back checked? All this sounds like it may stem from the nerves from the spine. I would see an orthopod spine specialists or neurologist. Read more...