Ive seen this come out in one of my hens.She is fine now.I noticed you said you were changing there water source?Perhaps she couldn't figure it out or was suspicious of the new water source and didn't drink out of it which led to a crop full of grass with no water to help mush it up.There are many scenarios that may of caused this.The best you can do is watch her and make sure she isn't exhibiting any other symptoms.I would buy her a bag of grit to help digest the grass properly.I have never used grit before.Only because i have a crick cutting half way through my property which supply's them with small stones.Best way to tell that there finding grit when free ranging is to check there crop at night.If its hard there not.