It's probably your shoes.
Otherwise, trying to support the weight of your body on your forefeet would cause excess strain in your calves.
This is from "How They Train", by Fred Wilt, 1959.
"The outer edge of the ball of the foot makes first contact with the track. Immediately thereafter (the smallest fraction of a second later), the heel touches the track without reservation and momentarily bears the full weight of the body as the runner rides smoothly forward for the next stride."
"It is readily evident that any running, especially sprinting, which does not permit the heel to touch the track with each step, robs the runner of maximum leverage. This explains why Percy Cerutty, coach of the great Herb Elliott, regards toe running (without use of the heel at any time during each stride) also as one of the two highest crimes in the running movement."