I have left knee pain and i've had no injury. x-rays and mri come back negative. what can it be?

Knee pain: Pain the knee can be due to tendonitis, plica, cartilage wear, synovitis, weakness, vitamin deficiency. Just too many possibilities without a proper diagnosis. If your knee hurts, start with a basic exercise program for your hips and knees and if your pain persists, or if you feel like the knee is giving out then i would see your orthopedist.

Tendonitis: Rest elevation antinflamatories and time. Soft tissue injuries that are minor can cause pain and do not have to be evident on radiographic studies.