FTM needs help !

My daughter is almost 4 months old. Her grandpa was watching her today while I worked. He told me she was gassy so he gave her peppermint water. He didn't call and ask me at all. I heard babies can't have water till there older? Anyways, she's having watery diarrhea (gross I know). Should I be worried? FTM any advice would help. thanks!