A Ph.d in anatomy asked me this question: Why is the expression "bodily fluids" and not "body fluids"?
In this context, bodily is a more precise word. A body fluid could also be some kind of fluid you put on your body (think body milk, for instance); a bodily fluid, on the other hand, is a fluid pertaining to your body.
Nevertheless, it is not true that body fluids is not used. According to wikipedia, both expresions are synonymous. And a Google search reveals that both expressions are used more or less the same (354,000 vs 395,000 occurrences).