Recently done blood work, ekg all good , no diet.. eat 1 hour before exercise and 10 seconds into my run i fainted. used to run 3 miles a day. cause?

Needs evaluation.: Your episode merits a careful cardiovascular examination, including a careful history, physical exam, including a check of your blood pressure, sitting and standing. If that proves normal, then a stress test and 24 hour (or longer) holter monitor test should be considered. If you have a heart murmur, then an echocardiogram would be included among the tests.