I totally agree with your spidey senses FireSprite. All the following around this woman has done, from months ago even, adds weight to this observation I think.
The fact that there is now a mystery child that she has to drop off at the same class as Mamabear's backs it up too. Someone's poor child has been dragged in to the drama.
I'd be very careful Mamabear, hell hath no fury etc - even though she isn't being scorned as such she obviously seems to be carrying around a lot of resentment and your ex is allowing himself to be pulled all ways.