Arm pain occuring only to the one side!

[semidead]

Hello there!
I play piano unprofessionally, and I sometimes experience arm pain, although I am very careful about having very relaxed all the body and watch over my posture.
The strange part is this:
That whenever I experience pain (which is not excruciating, it's just preventing my hands having a sense of flying over the keyboard) it is always on ONE side. Never have I the same effect right and left simultaneously!
If I have it on the right, the left hand is perfect.
But now it has shifted, my left hand is heavy and the right one is light!
Any ideas?
Thanks

[stagefright]

slow down
you are probably playing to fast for your state of technical ability
excercise slowly and gardualy increase the speed
the speed will come by itself without muscle pains