A classmate kissed my baby on his lips!!

Ok so I'm doing my masters and me and couple of my classmates decided to go out for lunch. The program started this fall so I barely know any of them. I brought my baby with me because my husband is travelling and while some held him (which I don't mind) one of my classmates kissed him on the lips! I was SHOCKED! I'm his mother and I have never kissed him on the lips. I was actually talking with another friend so I wasn't really paying attention but I caught her during my conversation. I'm so angry at myself right now for not saying anything I'm so mad at her but more at myself. She is from a different culture so this might be normal for her but I certainly didn't like it. I know I should have been clear with her but I wasn't. I don't know why I'm posting this but I would love if you guys can give me some advice on how I should react in these kind of situations without being so crazy and rude.