I know what you are saying when a player "chokes", but that is usually pretty derogatory. That assumes that the player should have done better and did not. And to say a player is "choking up" while batting, meaning she didn't come through in the clutch, is pretty tough when you consider that the most successful players hit the ball only 3 out of ten times. (That's Hall of Fame on the professional baseball side.)
In most cases when you hear someone say "choking up," like: "She is choking up", it usually means that the player is "choking up" on the bat, meaning they are move their hands more towards the center of the bat (closer to the top of the grip of the bat) as opposed to not doing well in a particular point in the game. This will make the bat seem lighter as you hands are mroe centrally located on the bat.