Exercise, sometimes feel faint, other times fine?

ive kept fit for the past 10 years and have had no problems. i could do hard-core workouts and feel fine - up until two years ago when i started to notice that i was at times feeling woozy and lightheaded and generally weak, like i was going to p out - on and off, but over two years it seems to be more and more 'bad' days like that if i exercise. it has scared me into slowing down the intensity and frequency of my workouts, but even still, if i do take it easy with exercise, i will sometimes get this feeling anyway, and have to cut short my workout, whereas other times i feel absolutely fine. is this anything to worry about? im 43 and as far as i know, good health. maybe its just old age? what other factors could be at play? could it be my heart? bit scared im gonna drop dead one day on the treadmill... u read stories bout sudden death after exercise and you think f**k, is that gonna be me? i dont feel like this if i dont exercise, so whats up?