Why do i have a knot where i once broke my foot?

It could be: A bony exostosis or spur that formed. You can have some scar tissue that formed. Is it painful? One needs to also rule out the possibility that you happen to have a soft tissue growth that happens to have occured in the same place as the injury and that the two are unrelated. This would be important as you would have to ensure this process is a benign growth and rule out anything serious.

Sometimes a bone: Will heal with a bone callus. This is a thickening of the bone across the fracture site. Depending on how much time has elapsed since the fracture, it could be that and/or some residual swelling. Some of the bone callus does remodel/decrease over time.

Knot: While it is usually bone callous as stated , to be definitive an x ray and exam are required . See your podiatrist .