Makon has had to have xrays and U/s the last one we told him the doctor wanted to look in his tummy and that he needed to lie still while they were!

perhaps you could say some thing similar? "the doctor wants to take some photos of your tummy and to do that you might have to lie down" if you have some xrays around maybe show her them! 'see these are photos of mummys/daddys -----! they are funny looking photos"
make it an adventure rather then a scary thing try not to mention its to help her poos!