There are many possible reasons for knee pain.
One common cause is misalignment in the feet arches and ankles which create an imbalance, causing stress and pain in the knee joints. Another is an imbalance of the pelvic region, including hips and sacrum, which cause the body to protect itself by shifting the majority of your weight to the other side. The structural imbalance, caused by either of these reasons, results in spinal nerve stress and muscle spasms on one side of the body and will pull one leg up short.
When knees are out of alignment, they will often "click", "lock", or "give out" similarly to meniscal tears so it is important to have a chiropractor, who works with extremities (arms or legs), differentiate the origin of your symptoms.