How long you been boxing for, when I first started (and some times even now days when I'm being sloppy... ) if I went to through a hard straight shot (usualy only happened to my left) if I was out of range I would push to far and lock the elbow past its safe range of motion, which hurt...

Could just be something like that, your hitting to soon with your hook and not locking the elbow or something, how long you been training, could just be because your not used to it.