Your thumb strap isn't right. Also, you may be holding your hand in the wrong position. If you have one of Marc Savoy's videos he explains the way to use your thumb. One should be pushing with the palm of the right hand, really relieves the pressure on the thumb and makes it easier to move around on the keyboard. On the high notes (closest to the floor) one if forced to use one's thumb to push but on the middle and low notes it is easier to use your palm.
thanks Hugh for the tip about the palm.
Nedro, If I'm right, what Hugh means is that you don't actually place the palm up against the box, but use it to give some back-up force to the whole right hand in general, including the thumb. correct me if I'm wrong Hugh.
both you guys have cool websites too!