Cooking chicken breast-it shrinks and becomes rubbery!?

I am good at so many different techniques as far as cooking yet I seem to have trouble with chicken breasts. I will pound them out to about 1/4 inch, and then when I put them into the skillet, its like they immediately become practically the same size they were before...its like they shrink and become thicker. I have noticed this also makes them a bit rubbery. what is the problem? skillet too hot? hellp