The only valid response when you simply cannot describe the pain which you are experiencing. Sometimes it's because you can't get enough air to really vocalize it, as when you get hit in the stomach and you can't scream because there's no air - there may be just enough air to say, "It hurts..." before collapsing.

Sometimes the pain is so unbearable that you cannot even scream - rocking-yourself-on-the-floor-in-a-fetal-position-with-tears-streaming-down-your-face type of pain.

Note: the pain being experienced need not be physical. It could be emotional, as a response tor just being told that your SO or child died.

This is the type of pain which cannot be expressed because there is no word or phrase which adequetely explains it, yet it is still communicated by the understated phrase, "It hurts", and the way your body and face are grotesquely deformed by your body's involuntary reaction to the pain.