There do seem to be bugs going round that have no symptoms other than pain/cramps.
One other option that springs to mind is that it could be psychosomatic. When I was young I suffered continually with stomach aches for which the doctors could find no physical cause. They said I was naughty and I was faking, but to me the pain was real. I hated school, and had really problems throughout my time there. Later I suffered from depression (from age 11) and stopped going to school.
I don't know if it could be anything similar with your dd, or even if all this could be any use to you at all - if it was, I'm not sure how you'd go about determining whether that was the cause or not.