Yes it could be anything. She may have a very sensitive tummy like me. I eat something and it always hurts. Sometimes the only way to make it stop is to have a empty tummy. Nah, it it could be acid reflex. I really don't know, just throwing out some ideas.
The pain can be from a number of things.. like salmonella, food poisoning, etc. Just have her rest up and drink enough fluids. If the pain continues after another few days, then make a doctors appointment. Good luck