@KoiwiGal - The thing is, I would rather be safe than sorry. I know it can be expensive to go to a doctor, and maybe if you can't really afford a casual visit, you're better off monitoring it, but if I got hit hard enough to suffer that much pain, I'd rather just get checked out.
I think if you do serious damage to your kidneys, waiting for more than a few hours to see if it will come right on its own could be a big mistake. And bruised kidney symptoms might also be something else that's gone wrong.
Of course, the best option is not to get into that situation in the first place.