I had sort of similar.
Broke up with my ex for a few weeks. During that time he went on OLD and met a woman. Was a bit more than yours- around 3 dates, they had sex and he had her over to his place too.
I had no idea about this and got back with him after the "break" after we discussed things (he came begging not me).
Anyway. It came out later as this woman started sending me messages all over social media (I have various pages personal and business).
I was very upset, even though we were broken up, for the reasons you have already stated, I felt like I was very easily replaced. And all the things he said about feeling down, needing an ego boost after the breakup etc. made me think a bit less of him like he couldn't be alone for 5 mins!
If I'd known what happened before I took him back, I never would have. I felt really violated.
She also sent me lots of nasty messages (not sure what I had done though!). So I was angry at her too but didn't respond to it.
In the end I ended things with my ex as I never really got over it. And as time went on I thought more about how he treated the other woman, basically used her then ghosted her and I felt less and less of him.
I do think of her as a bit "unhinged" as she still stalks me and my friends on social media to this day. Can't get my head around it as it was years ago and I'm not with him. But I felt sorry for her too, I think it can really hurt and mess with your mind when someone love bombs then ghosts you.