I have a boxer cast on for fractured metacarpal five and now my middle finger has a tiny knot on it at the joint from metacarpal to phalanx.?

Hard to say. It is hard to say without directly seeing. It could be potentially related to having to wear a splint or cast for the 5th metacarpal injury. It could possibly be another condition associated with your original injury. I would make sure your surgeon is aware. An exam and possibly a followup X-ray of that area would likely show the cause.