The hard lump that is just below the right kneecap may be a condition known as prepatellar bursitis, which is when the small sac that is located in the front of the kneecap is inflamed. The bursa is a sac that is composed of thin and slippery tissue that functions in helping skin, muscles and tendons slide easily over the bone. The prepatellar bursa, in this situation, is the sac that helps the kneecap slide freely when the knees are being moved.
The bump below the right kneecap generally indicates that the prepatellar bursa is inflamed. This can happen if there is a direct fall or blow onto the kneecaps, and the kneecaps bang onto the bursa lying underneath. The impact may result in bleeding in nearby tissues that would eventually drain into the bursa causing an enlarged lump to form. In addition, the blood that surrounds the bursa is also thought to be responsible for creating an immune response, which causes the walls of the bursa to thicken.
In time, if the bursa is repeatedly the subject of impact, the walls of the bursa may thicken permanently. This may result in a permanent hard lump under the right kneecap over time.