You may have a tear in the articular cartilage of the knee. This is the fibrous layer of cartilage that covers the end of the femur (thighbone). The only way to know for sure is to have a physician examine you and order some imaging studies.
Weight-bearing X-rays with the knees straight will be taken. Then several other views with the knees bent 30 and 45 degrees are viewed. MRI is the best imaging test for this problem. The signals help identify where and how deep the full-thickness tear has occurred. The MRI also shows the condition of the bone underneath the cartilage.
Depending on the location and size of the tear, you may or may not have symptoms. Or you may have very mild pain and symptoms even when there is a serious tear. Some patients only notice pain when the knee is loaded at a specific range of motion. Symptoms of locking, catching, and clicking with motion are common.
If the tear or lesion is present within the weight-bearing axis, pain occurs only when the joint is loaded at a specific angle. This may be what you are experiencing. It's best to have a physician examine you. Left untreated, these types of injuries can develop into arthritis.