Why do I keep thinking about this girl I don't like anymore?

I've liked this girl for nearly 4 years I was close to dating her once but never did, but about two months ago I stopped liking her completely because I was tired of her and liked this other girl way better. But for the past two weeks i dont know why but I keep thinking to myself what if i still like her, what if the girl i like now isn't better than her, what if I'll start liking her again, and I can't stop checking her social media still even tho I only do it like once a day if even now, and it's really pissing me off because i know i dont like her anymore because i literally barely thought about her for two months and didn't care about her at all or our past and i like someone else now way better, but for some reason I keep thinking about these things in the back of my mind. Anybody know what this might be?
Why do I keep thinking about this girl I don't like anymore?
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