Am I overreacting? Husband (separated) now dating his Ex he told me not to worry about!

My (32F) separated husband (30M) many years ago introduced me to a “friend” of his. I agreed and we started hanging out with her, her then partner, and various other friends who would be there on and off. One night I walked into the room to find my husband and he and this old friend abruptly stopped talking and were laying on the bed next to each other… it felt weird. I went on with the night and she just HAPPENED to drop the fact they dated when they were much younger. I was furious, I didn’t mind the friendship, I just was so annoyed that he had lied to me about how he knew her. We then distanced ourselves from this situation because I was no longer comfortable. Several years passed and in these years we got married and had 2 babies. Every time we’d hit a rough patch I’d noticed messages from her and it was mildly flirty between them. I’d always bring up how I wasn’t going to tell him to stop talking to her, but I would reiterate that it made me uncomfortable and would hope that he’d stop knowing how it made me feel. He’d always say he’d never cheat on me and reassure me it was nothing, She is just an ex who was “crazy”. I would say if it’s nothing and she’s crazy, why do you need to keep her in our life? and it would die down again. Flash forward 6 or so years… we have separated. This came about as him kicking the kids and I out of our home. We were going through a big struggle, and despite me trying to get us help and get on the same page one day he just snapped and booted us. It’s been a few months now, I live across the road from him with my parents. THE EX SHOWS UP AT HIS HOUSE, she then sees our neighbour and my dad standing out the front, panics, and does a lap around the block. My dad relays this information to me and I go outside to see my Ex and his Ex getting into his car and driving off. So dear people of reddit, do you think it is reasonable for me to believe that this was probably going on for a while, and even if he wasn’t physically cheating that it’s more than likely he was emotionally cheating until he was ready to boot our kids and I out of the house? I am over reacting or is this suspicious and shitty as hell?