Im 19,yesterday at the gym on the treadmill while walking i get lightheaded feeling.when off everything normal.normal blood and holter moniter.cause?

Hard to say: Feeling lightheaded is a non-specific symptom that can mean many things. Poor conditioning, low blood hemoglobin content, low blood sugar, early sign of illness, etc. . Since you appear to have had some studies that rule some treatable issues out, you will just need to keep track of your symptoms & followup with your doc if it continues.