Several things can cause numbness to occur in your right arm such as compression of the nerves, loss of blood flow to the arms or even damage or irritation to the nerves in your arms.
My right arm or even left arm will sometimes become numb in the morning if I happen to lay on either arm during the night when sleeping then the numbness goes away after awhile.
So it could be that you've accidentally compressed a nerve in the arm which could be causing the numbness.
If the numbness happens too often then you might want to see a doctor about it as it could be caused by diabetes as well although most times symptoms of diabetes will occur in your feet first.