@nononono,
You should go to an orthopedist or podiatrist and have it xrayed.
When I fell and broke two vertebrae and dislocated my pelvis, I thought that was the extent of the injuries except for a minor sprained foot. The back and pelvis got better, the foot got worse.
Many months later, I finally had the foot xrayed and it turns out I also broke my foot in two small places, one of which is the joint that connects the heel to the main part of your foot. The arch was always sore, especially when walking downhill. The other spot was the joint that connects the small toe to the main foot. Neither of them were properly set and immobilized at the time of injury so they didn't heal properly. Now about all they can do is give me cortisone injections in the foot.
You may have an unknown old injury that is acting up. If that is what it is, a cortisone injection is miraculous.