Why do my middle toes go numb when I work out?

…specifically the middle toes on my LEFT foot?! Right foot is usually fine.

I did just buy a pair of new gym shoes (New Balance 1063’s, with a roomier toe box than the other four pairs I tried and returned), and had them fitted at a great local athletic store. They tested out the toe room and said it was perfect. I had the same problem with my previous shoes. I really only notice the numbness after about 20–25 minutes of working out, and it only bothers me when I’m on the elliptical, jumping around in Kickboxing class, or running outside (not on the treadmill). Any thoughts?