Mom keeps overstepping the mark

Since I got pregnant my mom has been overly excited, going out buying everything for the baby that I wanted to buy. I got over it eventually as I know if I say something, she will be a martyr and refuse to help us anymore. 
She's currently in Canada visiting family (we live in the UK). When she skyped me yesterday, she told me that her and her cousin had gone out and bought MY baby's christening/baptism dress!!! That's something I wanted to buy for my own child. But if I ask her to take it back she'll get upset. If she brings it home I have to use it as she will expect me too 
I'm so angry and hurt that my mom would go out and buy something so special, and take that experience away from me