I have demo eye glasses with clear glass. When I wear them, my eyes hurt. Is this normal?

It can be. If you are in need of prescription lenses and wear clear glasses, you may be straining through them. Alternately, the plain lenses may actually not be of very good quality and may be warped. This can create a "fake prescription" in the lenses that is just enough to affect you. You are better off not wearing the glasses.
Probably not. The demo glasses may have zero power centrally but may induce some distortion at the periphery. Truly optically flat lenses should have no problem with this.