Did he use me? Did I do something wrong?

My ex had were dating for an about a mouth and a half. He was my first boyfriend. I loved and cared for him deeply and I thought the same was for him to me. That was until he broke up with me saying that he felt that we weren’t right for each other I respected his wishes thank him for the time and experience that I got from the relationship and I asked if he still wanted to be friends or just not talk at all and he said he didn’t want to talk at didn’t really think anything of it and moved on. That was until the next day when he posted on his snap stories that he had a new girlfriend and I thought to myself there is no way he got one that fast. I never really saw him hanging out with other girls at school but I also knew that he was friends with a lot of girls and I don’t blame him and I didn’t care. So either A. He used me to get back at his ex before me to try and make her jealous. or B. Cheated on me because there is no possible way that he could get a new girlfriend in less than 24 hours if he hadn’t cheated on me. What do you sits think. Anything helps. Thanks so much!!
Did he use me? Did I do something wrong?
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