I don't do the reverse tomahawk myself, so read my suggestions with that in mind.
Make sure that your arm movement is "extending" from the elbow, not "twisting". I see the elbow twist a lot when players first learn the backhand loop, resulting in pain like what you are reporting. The reverse tomahawk may be performed by a similar twist, but the opposite direction. Torsion around the upper arm may give RSI type ailments in shoulder and elbow.
Footwork is important. A service stance with your right foot more forward (if you are right handed) may give your arm a better angle towards the ball, so you twist it less.
That "whip motion" mentioned above is probably more essential, also to help avoiding the upper arm torsion. You need the rubber to really "dig in" and grip the ball.
Ummm ... not sure that I like the way that last sentence came out. Maybe the time of day