Today I did my first cardio training since football last year. It showed too. I warmuped up with some dynamic strecthing, jogged about a half mile, did sprint work for a mile, then walked a mile and a half cause my core was cramped up bad, than ran up a hill and finished with a walk. My legs were fine, the strechting is really paying off, but my core was all twisted up. Could this be from a weak core? I haven't done any direct ab work in a long time. And i had a good breakfest, protein shake and oatmeal, so i dont think it was my nutrition, and i was carrying water with me. Any ideas?