Dealing with the Biomom...

We get a random out of the blue text this morning from my fiancé’s ex(super high conflict) ranting at us at 7:30 in the morning about how “ don’t tell or encourage the kids to call “my name” mom. I am their mother. Not okay.” And first thing, neither my fiancé or I have said or done anything of the sort!!!! We haven’t even had the stepmom conversation with the kids yet and she’s coming down on us out of the blue for no reason!!! I have no idea how to deal with this level of crazy and am lost for how to process or deal with the situation in any kind of way. What the fuck happened in her world to make her lash out like that??? Because it definitely wasn’t something we said or told the kids. In fact, whenever I’m talking to the kids about her, I make sure to say mom. Stuff like “you’re going to moms house” or “you’ll have to talk to your mom about that, it’s going to be her night”. How the hell do I deal with this situation?