Anytime i try heavy exercise like workout dvds etc i soon start to get pains in my stomach like cramps only ever when i exercise what is this?

SPASM: Most likely it is a spasm in your abdominal muscle from exercise you could take nsaids like Motrin or aleve (naproxen) before the exercise in spite of your young age make sure you are not confusing abdominal pain with chest pain or palpitation since your heart rate has to be up with exercise if persist or if it is associated with dysiness or cold sweat see your pcp.