Ex introduced girlfriend to my child without telling me..

I have a 2 year old daughter with my ex. We had an in-depth conversation that if either of us had new partners to notify one another so we know who our kid is around and I was pretty firm on that because it was important to me.

I took my daughter to the water park today and she instantly ran up to another little girl the same age and started playing as if she knew her, I figured it must be a preschool friend. The mother then came up to me and said “hi you must be “___” mother, im “___”, “my exes” girlfriend, they’re such good little playmates!”.

I was like wtf??? I was pleasant but pretty shell shocked. Then the mother got the little girl dressed and she was wearing my child’s clothes!!! (Distinct clothing I bought for my daughter)

I am just seething with how disrespected I feel and need some advice on how to approach it.