Besides heat and caloric intake, what would cause someone to continue to faint during rigorous exercise? could it be i'm just out of shape?

Fainting: You may have become anemic from your vigorous exercise. See your doctor for blood work.

Heart disease: If you are truly fainting during exercise, and not when standing in between exercise bouts, you need a cardiac evaluation. Fainting during exercise is an ominous presentation of a variety of cardiac diseases which can be fatal. Depending on your precise history, you may need an ecg, echocardiogram, exercise test, and possibly other evaluation.