That's a tough one, Karma. Lots of possibilities.

You may have injured something as in pulled a muscle or strained a tendon. Might want to go easy on the miles or even check with a doctor.

Your seat may be set just a bit too high. I find that this puts more strain on my feet. Too low, of course, is a way to blow your knees. On really long rides, I sometimes change seat height as needed to reduce pressure where I'm getting pain. Might give it a try.

Most of us have one leg longer/shorter than the other and this can place more strain on one leg/ankle/foot than the other over long stretches. My left leg is slightly shorter, so I experimented with the cleat position on the left foot, moving it just a tiny bit forward compared to my right. Seemed to help.