My wrist hurts when i bend it back. is this a sign of a broken wrist?

Depends: The information you provided is not enough to make a diagnosis. Was there trauma? Where is it painful? How painful? What type of pain? Etc. Etc. I recommend that you be evaluated and x-ray done if you think you could have broken it!

Not necessarily: If there was not any acute injury then you may be just trying to move the wrist greater than what it was designed to do.