Prior to clipping in, I had the same problem. I was told by several folks that is was due to the placement of my foot on the pedal. If your downward stroke pedal pressure is focused on the area directly under and in front of the ball of your foot (sesamoid bone), kind of like where the "knuckles" of your toes are, you are applying pressure to the nerves that run between each toe in the soft tissue located there. With pressure on this area, the result is numbness. Proper placement of your foot on the pedal, and wearing stiff soled shoes might help. But I caution, this was my own experience. Your might be different.

I agree with KY that researching/reading/discovering is best. Good luck