We saw the chiropractor who diagnosed a common ailment in children: "Acute something-or-other". I did know the name at one point, it has two "l"s in it ... Some tissue pinched between some vertebrae leading to inflammation and pain.
The upshot is ice and ibuprofen, watch for fever, and give it a couple days. Which is as much as I expected, but the experience was much more pleasant than waiting for our/a doctor to find time for us.