Neurology? I used to get that really intense tingle when we went down a steep hill (why? I have no idea, but I do know I wasn't the only one, because so did my best friend and her sister) ... Some girls seem to just get nerve firings "down there" (clitoris?) more often or more intensely ... some of us don't ... sometimes it goes away or gets more as we grow older ...
I could see pressure from a morning-full bladder triggering the nerves in front of and below the bladder, in a way daytime-full (not as full) bladders wouldn't ...
anyhow, I doubt doctors have a test for that, especially for little girls, but you could kinda watch her symptoms and see if it fits ... might not relieve the problem (so to speak ;) ), but at least you'd know what it might be.