this could be a lot of things, but if you're willing to experiment a bit you could narrow it down.
first, stay off the treadmill for sprints. walk, jog, run on a treadmill if you're into that kind of thing, but you can't get a legitimate sprint going on a treadmill.
if you're running on grass, ditch the shoes. you can go with something barefoot if you'd like (such as the vibrams), but i think barefoot is more fun and feels better.
don't stretch, or stretch less. a good warm-up and cool-down is better than any stretch you're going to do. and, make sure that any stretch you do is dynamic.
give those things a try and see if they help...knowing full well that they may not. you can work a lot harder on grass than you can on a treadmill, so that may be contributing to the soreness. and, it might just be a little arthritis or something. regardless, keep sprinting.