Your problem is most likely related to your diet. How long is it between your last meal and your running? If you're having lunch then heading off to the track an hour later, you might try to change your schedule so you grab a quick sandwich after running instead.

What you eat can also be a factor. Try to stick to carbohydrates before running, and save the proteins for afterwards (i.e. don't chow down on a burger before cross country practice).