Pain in back of Hand

[Tom16]

I have been using captains of crush grippers for about 2 weeks but have now stopped because i am getting pain in the back of my hand, the pain is around the area that the bone in line with the index finger is. There is no pain when i am not using my hand but whenever i grip something, especially the gripper i get the pain, my hand also feels weaker. Does anyone know why this is and what i should do about it?

Thanks in advance