Why is my ex’s girlfriend or ex girlfriend who knows if they are together now or not but why every time she sees me she gives me her middle finger?

I haven’t done anything to her nor I don’t know if my ex told her anything but why does she not like me at all. Everyday she sees me she does it even though I can’t do anything because I’ll be driving by. It’s starting to piss me off. I can’t like I don’t see her but she’ll be stare at me and cussing too.
Why is my ex’s girlfriend or ex girlfriend who knows if they are together now or not but why every time she sees me she gives me her middle finger?
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