Could be something going on with your circulation. Since you've already tried getting better shoes, I suggest 1) try a workout on the treadmill and see if the same thing happens, and 2) go back to the shoe store where you got fitted to see what they might suggest. If neither of those work, go visit your doc to rule out anything serious. I had this happen to me before while doing some at-home workouts and it was my shoes :\ Hope you get it figured out soon! Keep us posted.