Why could one of my feet be swollen when I have no injury to the foot?

Veins . There could be a problem with the return of blood up the leg in the venous system.
Is their pain? I am not sure of your age but if you are young as I am presuming from your icon then you may have a stress fracture which could happen from a vitamin d deficiency as opposed to you recalling some history of trauma. This is one of may possibilities.