How would you feel if your MIL did this?

On the weekend my partner and I went shopping with his mum. When we picked out something (quite expensive) to buy for our daughter and she took it away from us and purchased it herself. This upset me so much. I don't know why she did it or why it hurt me but it did. It upset me so much that when I was telling my partner how I felt it made me cry, and I don't cry easily. She was also saying how she was going to purchase a high chair and a stroller "to keep at her house" as if the ones we have for her aren't good enough. We don't go to her house often so there is no need for her to have a highchair, we managed to convince her to buy one of those feeding seats that just attach to the regular dining chairs. There is absolutely no reason that she would need to buy a stroller, my daughter already as a brand new one that she was actually using WHILE we were shopping. She made a comment about how her granddaughter should "only have the best" but there is aboslutely nothing wrong with the one she has! It isn't one of those fancy ones, it is lightweight and very easy to push around, unlike the expensive ones she was looking at purchasing. My partner talked her out of it but it just made me so mad. I don't think she understands that it is OUR turn to be the parents. This just really upsets me but I can't help thinking that I am being silly. How would you react if your MIL did this?