Ignore it. Pain can't hurt you any more than your injury already has, you there's no need to acknowledge your pain. The worst thing you can do to your enemy is ignore it.
If the pain is too much to ignore, even by distraction, try causing more pain. In a different location, it causes you to forget about the larger pain and may make you feel better by causing symmetry (if the pain is in one side of the body, mirror it). In the same location, increased pain gives you perspective. In addition, increasing the pain of the injury can cause your body to enact anti-pain measures, resulting in substantially less pain than there was originally.