Somebody please explain blood types!

I just gave birth today to a beautiful baby girl. They explained to me that she has blood type ABO incompatibility.... I have NEVER heard of this in my life! I am O positive blood type and her father is O negative. It is possible that our newborn can have an ABO incompatibility blood type? I’m totally confused because I thought that if the mother and father were O blood types that we could only produce O blood types. Plus we have a son that is O. I haven’t had sex with anybody other than my husband so this would really be rare and strange. Please somebody help me understand!