Devastated and angry

I did wrong by cheating on my husband last year. We split and both of us have new partners. (I stayed with my affair partner). Today my son told me that there was a card (We had fathers day in the uk) from my ex's girlfriend. It said on the front 'Dad, You're the best mum 'And inside it said 'To Jon, No words needed. I love you so much. Maddie xxx
I am devastated and angry that she would take it upon herself to do this. How would you approach it?

She meant that I am a crap mum and my ex fulfils the role of both parents as he is so wonderful 😐