Why did she leave me for her ex?

She broke up with me last Sunday, saying she couldn’t handle the relationship anymore. Before I left, I asked if she still had feelings for her ex-situationship. She admitted she did, and that was the main reason for our breakup. She immediately started reposting about wanting him back and even reached out to him. I did my best to be a good boyfriend, especially since she told me he had treated her badly and cheated on her.

Since we share the same friend group, we’ve seen each other a few times but barely talked. Yesterday, at a birthday party, we finally had a proper conversation again. We teased each other and laughed a lot. The way she looked at me made me feel a little spark, even though she kept talking about him to her friends. Unfortunately, we also ended up making out, which has left me feeling confused.