AIO?

This might be long but bear with me. My fiance (m25) & I (f25) are getting married within the next month or so (court wedding) and we’ve been through a few ups and downs. We’ve been together for 3 years and I found out like 6 months in that at the beginning of our relationship his ex followed and dmed him on IG. When I found out he told me it was just a few times that she would dm him and ask how life was going. Then try to bring up shit from their relationship like “oh I still have this from that one time…” or “I still have this that you bought me.” Mind you, this would be after he told her I was with him. He would shut it down but my thing is, why even reply back to begin with? I really took this very hard because he only knew his ex for 2 1/2 months at most and all they really did was have sex which is what he explained the relationship dynamic to be. They “broke up” because she cheated on him and in my head, I would assume that if someone you didn’t know that long that did you dirty like that contacts you, you would ignore them?? Recently I asked him to be completely honest with me about any and everything he may have done throughout our relationship so that we can get it off our chest before we go and get married. He admitted to me that he downplayed the times he spoke to his ex and instead of it being a handful of times, it would be every 3-4 days for 4 consecutive months. I asked him what they spoke about and why he even entertained a conversation with her for so long and he said that she would dm him and ask how he’s doing so the conversation would flow but it never turned sexual nor did it turn into a reminiscence of their “relationship.” I can’t help but feel angry and upset that he kept this from me but also wondering if he is actually telling the truth about the conversation only being platonic. I’m not sure if I am overreacting because this was so long ago & I don’t want this to ruin what we have because in all reality, she’s irrelevant. AIO?