Disappointed and hurt

My June baby will be 6 weeks this week. At 3 weeks, he was diagnosed with colic and it has been quite the ride since. After doing some reading, his father and I decided it would be best to change his formula as the one he was on didn't seem to give him any relief. We suspect a cows milk allergy.

Yesterday, I asked my mother if she could watch him while I went to buy the new formula. I had only left a bottle with one ounce of formula since he had eaten right before we arrived. I knew that I wouldn't be long.

When I got to her house, both she and my stepdad had a look of panic. She asked if I had any milk. I thought that the question was odd and told her no. She mentioned needing 'milk' for my son so I immediately looked at my sons bottle. It was full. She felt that it was okay to give my son 2% milk with a little bit of water. She didn't realize that I would be back so soon (although not soon enough).

She thinks that it's a big joke, that I'm upset about it. When my brother got home from work, she was proudly telling him what she did. She even tried blaming it on my stepdad. She also said that whenever I leave my son there, she's going to continue to do it. So I told her he won't be over there unless I'm there with him.

Am I wrong for feeling this way?