I had surgery on my left foot 7 years ago. i wait tables for a living. my left foot recently developed a small knot for no reason. it doesn't hurt.

Bone spur: Most people have foot surgery secondary to issues with the bones or muscles/ligaments. In the process, often times, a bit of bone may be removed or redirected. Over time, the resected bone, or the redirected bone can develop bone growth. Also, you may have a fluid build up from the joints causing a small knot.

Knot: You did not state whether the knot is in the surgical site or not . There can be many places on the foot for this to occur related to the surgery or not. See your podiatrist or the doc who performed the surgery for a diagnosis.