Deafness in ear?

For a while now, at least a year or two, when ever I'm active, my left ear goes completely muffled. For example, if I ran a mile, my left ear would, literally just go out. Everything sounds as if theres a pillow over it. It seems that when I make loud grunting sound, it goes away for a couple seconds, and comes back.
It ONLY happens when I'm active though, and after I relax completely, then it goes back to normal. So what's wrong?

[EDIT] It doesn't happen any other time other than me being active. Not during orgasm, to when I'm nervous and sweating. Just when I exercise.

Could be ear wax which moves around. Try a few drops of olive oil in the ear to loosen any wax. Sometimes I wake up and one ear is deaf, but it goes as the day goes on.

Well obviously if you move around so does your ears and any wax in your ears. If its not ear wax it could be a small stone or insect or something which moves around when you do.
Or it could be air or fluid trapped behind the ear drum, much like you get after going up or down a mountain or in an airplane or after swimming. Try moving your jaw around to try and release anything.

It could be muck left over from an infection or cold that hasn't had a chance to escape.

The ear has a number of cavities in it that may cause things to get trapped. And depending on head orientation and movement may shift it around causing temporary deafness.

If you are concerned about it see a doctor for an ear check he or she will take a look.