So confused

So today I went to drop my daughter off at 4k. It was my 4 year old and 12 week old and you have to wait out front until the teachers come outside to greet you then they take the kids to their classes. Well while we were waiting outside there was another mom with her daughter and her baby. I think her LO was around the same age as my LO and I said to my daughter "look how cute another baby like your sister" the mom looked at me and glared and turned her LO the opposite way so he was facing the wall of the school. I am not sure what I did I feel bad if she was upset but if another mom said that about my baby i probably would just smile. I am just so confused at what I did. Did I do something wrong?