The knee is a complex joint because even though it is intended to only move in one plane--unlike, for example, the shoulder joint which has much greater flexibility--restricting the mobility of the knee is important for a stable and steady gait. As you are experiencing, when the knee becomes unstable it can make walking or running very uncomfortable.
The knee is, in fact, supported by multiple ligaments and tendons which help provide the stability required for walking. Weakness in the joint can sometimes be a symptom of an underlying problem with one of these ligaments. However, it is also very unusual for this to happen without some kind of inciting event or trauma, whether that be a skiing or soccer injury or a car accident.
Overall, it sounds like you have had a recent change in the stability of your knee, so the best thing for you to do is see your primary care physician
. He or she can ask you about any recent events that might have led to an injury as well as examine your knee for evidence of inflammation, swelling, or joint instability. Physical exam can help identify whether there is weakness or instability in the various ligaments and tendons of the knee. Depending upon the results of this exam, your physician may send you for imaging studies (such as MRI
) and a consultation with an orthopedic surgeon