It could be a minor sprain, other usual cause could be stress hair line fracture which can only be ruled by an X-ray or a CT scan.
Next Steps
Apply a elastic crep bandange, keep your feet elevated, don’t put pressure on the leg, consult an orthopaedic for further evaluation. If it is a minor sprain, it will subside by itself if given rest to the feet.
Health Tips
But do not delay, consult an orthopaedic to rule out fracture