Remember this woman is angry with you and wants to hurt you. The angrier you get at her, the more justified she will feel if she is having an affair. She knows that turning up like that will wind you up, or, maybe she is trying to show you what you are missing in a 'hah' kind of way.
If she is seeing another man it is very hard to set boundaries once you are separated. Your concern is the children. You could set a boundary to do with the phone. Next time she says something like that you (very innocently) could ask how you should contact her if there was an emergency regarding the kids.
You could even compliment her on her outfit or perfume... that would totally throw her off track and one of her complaints was your lack of attention. Or, just ignore her and don't take bait.
P.S - She isn't indifferent; she has broken up her family. She will be struggling. Her anger and actions like you described in wearing those clothes is partly a defence mechanism in trying to justify her actions.
P.P.S - great news on the fun run outing today. I think you did tremendously!
