because your not relaxed enough.

I swim in the ocean, and i still have this. Try this: float in the pool, with both arms fully extended forwards, and both legs straightened behind. Float on your front with your head in water, after taking a deep breath. Slowly breath out until you have no air left in the lungs. Then wait a few seconds, to ensure lung-emptiness, then surface. If you have to lift your head before finishing, carry on doing the drill to build confidence. Also, if holding breath underwater trying to resurface, use slow movements that use less energy, to make the air last as long as possible. Good luck.