MIL gave baby water

I just found out that my mother in law gave my 3 mth old daughter water and I am livid. My husband's family are control freaks ESPECIALLY his mother. She once insisted on me going baby shopping with her and then seriously wouldn't let me pick anything out because I wasn't grabbing everything in the color pink.

I've noticed my 3 month old is having a way harder time with bowel movements. Like straining so hard she screams. Am I being overcautious or could this be from water being introduced too early? I'm trying to stay calm but her straining is noticeably worse. It sounds very painful when usually she would just grunt and squirm.

I noticed the change about a night ago. Me and my husband were staying at his parents for a visit with the baby this past weekend. I believe the water incident happened either yesterday evening or the day before. Has anyone had this concern with straining? Is it just something that gets worse as she gets older or could the intense straining be because of the water? Also not sure the quality of the water either. We couldn't find any bottled water for ourselves so hopefully they had one in their room we were unaware of and not just giving my baby tap or fridge water as the fridge water tastes surprisingly like tap water. How do I kindly tell my MIL that I'll break her hands if she interferes with my child's nutrition again? Should I take my child to the pediatrician just to be sure? We have an appointment in two weeks but man is the straining really scary.

Btw She's breastfed so I'm adamant about her getting absolutely nothing but breast milk until my pediatrician says otherwise and even then I'd still go with my gut on what's best for my baby's digestive system.