M.A.
It could very well be here iron intake or her calcium intake. I would take her to her pediatrician when you can. Don't get crazy concerned- just something they need to look into. Does she take any medication? I know as adults- things such as Pepto Bismol can do the same. It is a reaction to something she is tkaing in. Could be absolutely nothing. Are you still nursing her? Culd be something you are intaking. It is something reacting with the acids in her stomach.