I had an issue for a while with the outside of my right foot getting painful. It went away shortly after finishing the ride. It wasn't until someone commented that I discovered that I was kicking my knee out at the top of the stroke, but only under heavy load. I never realized I was doing it until then. I've learn't to stop doing that now and the problem has gone away. I'm sure there are many other explanations for your issue but this was the one I had.
OldBrit