What makes a little girl cry?

Last night all was well. Munchkin went to sleep close to her usual time without any fuss. Around 9:30 p.m. the crying began. She flips and flops around in her crib and begins her cries of frustration. Does she have a tummy ache? Did she have a bad dream? Was she angry that she was awake? I may never know until she can speak clearly....