What could have caused a lump on the side of your knee?

* Baker's cyst: A Baker's cyst is a fluid-filled sac that develops behind the knee. It is caused by an accumulation of synovial fluid, which is the fluid that lubricates the knee joint. Baker's cysts are often associated with other knee problems, such as osteoarthritis, rheumatoid arthritis, and meniscus tears.

* Ganglion cyst: A ganglion cyst is a noncancerous lump that forms on the tendon or joint capsule. It is filled with a clear, jelly-like fluid. Ganglion cysts are common on the wrist, but they can also occur on the knee.

* Lipoma: A lipoma is a noncancerous tumor made of fat cells. Lipomas can occur anywhere on the body, including the knee. They are usually soft, painless, and movable.

* Bursitis: Bursitis is inflammation of the bursa, a small sac filled with fluid that helps to cushion the bones and tendons around the knee. Bursitis can be caused by overuse, trauma, or infection.

* Tendonitis: Tendonitis is inflammation of the tendon, a thick cord of tissue that connects muscle to bone. Tendonitis can be caused by overuse, trauma, or infection.