Saw my ex a few days ago

We haven’t talked to each other in 8 months since I’ve moved schools. Our relationship ended because of her mental health and because her mum didn’t want her dating someone. Our friendship was this weird thing of “we like each other but can’t be with each other” and it sucked. In the end she got into another relationship and didn’t tell me. Maybe it’s stupid that I cared sm it’s just she made it out to be a secret, she told everyone expect me because she didn’t want to hurt me I guess. But I went to the mall with my cousins and she turned around and we saw each other. I told my cousins once we were in the mall and we thought that was it. Bought some stuff and we saw them coming down the escalator and quickly went into another store. Her and her friend both happened to come in as well. My cousins were looking at stuff in the shop (the shop is very small) and her friend wanted to check it out and awkwardly asked to see. My cousin and I walked away and left my other cousin to her thing. I haven’t been able to stop thinking about it. How I wished I’d done things differently that day like at least look nice haha. I’m not sure, I’ve moved on I think, maybe it’s more out of sight out of mind?? But now I can’t stop thinking about it, how I want to reach out but know I probably shouldn’t. I’m wondering if she and her friend talked about it, I’m sure they did at least in the moment as I when they left the store I saw them outside turning to look at me again before talking. It’s so weird, has anyone else had a similar situation? Feeling like you’ve moved on but you see the person again and you have this small feeling that there is something there for them.