Thumb pains

Hi!
When I ride my back I mostly ride not on the aero bars but with my hands up on the breaks. I've been training on my bike for about 2 months now (no prior tri experience) and my thumb is hurting when I press at the bottom of it. It's like the tendon has been damaged or something. I am also a tennis player and can't play tennis now. Has anyone experienced this? What do you do so that it doesn't hurt?

The pain that I experience happens when I hold something in my hand and it presses against the bottom of my thumb and gives me some kind of shock through my thumb. The pain is mostly in the top of my thumb though.

Sylvain