Bizarre pain! What on earth?!

So, today I was doing a hill run, about 3 miles, at a moderate pace, and I noticed a sharp pain in the side of my right foot. I thought I had something in my shoe; it felt like a thorn was poking me every time I landed on the ball of my right foot.

When I got home, I checked my shoe and sock thoroughly, but there was nothing there. I was stretching, doing some running in place, and I felt it even more sharply. The pain seriously feels like something is poking me in the foot on the right side bottom just forward from my heel. There's nothing there, but I would swear I was getting poked by a thorn or a pin or something. It only happens when I flex and land on the ball of my right foot.

Any ideas what the heck is going on??