Why is my "ex" keeping tabs on me?

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A girl and I really liked each other, and after mistakes on both sides, she ended up rejecting me - it ended up being a brutal ending to our "relationship." After 4 and a half months of no contact or even seeing each other on campus, one of her close friends is in my class this semester (my "ex" is not in any of my classes).

Here's the rub: her friend is ALWAYS watching me intensely when I talk to a new girl. Additionally, it's only getting worse because yesterday, my "ex" saw me talking to a new girl (yes, she did get very jealous) and her friend only got more intense the very next day. And no, I don't think it's because her friend likes me. Frankly, it's pissing me off because I'm trying to move on, but shit like this is keeping me from doing so...

Why is my "ex" keeping tabs on me even though SHE'S the one who ended it?
Why is my "ex" keeping tabs on me?
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