How do you deal with a passive aggressive ex?

I’ve been getting the cold shoulder from my ex for about 3 or 4 months. We see each other several times each week to hand off the kids. Today, one of my kids called me to ask if I had a certain book, and I could hear my ex in the background say that I’d taken the book in the divorce. (I don’t have the book, and I can’t ever remember having it.) She said it at a volume that was intended for me to be able to hear over the phone. I could tell it made my daughter a bit nervous.

Being bipolar, I’m not good at confrontations. To tell the truth, writing this makes me nervous.

Do I simply ask her what’s wrong? Do I ask her what I’ve done wrong? Do I point out the behavior and ask that it stop?

Do I do it face to face, over the phone, or by email?