Hand Ache?

[user]

I have been useing the powerball for about a week and everytime I use it now the muscle below my thumb is starting to ache very quickly and stops me from carrying on, it was great at first because i felt my forearm really tense afterwards but now I cant get to that stage becuase I have to stop becuase of my hand, also as i just mentioned, I seem to get a lot of tension on my forearm more than anywhere, is this right as the powerball is advertised as a wrist and bicep developer, thanks in advance for any possible solutions posted

Daniel`