What Causes Bump On Hands After Carpal Tunnel Surgery?

So my dad got carpal tunnel surgery on his wrist and after it was done he had to go through physical therapy sessions. Now he s been working it out and re building muscle at home. But now there s a bump on his hand that when you push on it, it goes away. But the more he moves his hand the more it builds. It seems to be a fluid but I can t tell.