Why was she following me around?

I went out with an old friend from high school last year, and we really hit it off. We had a thing for each other in highschool, just no one said anything.
After a few dates, she confessed it wasn't fair to me. She had recently been broken up with less than a few months prior and she still wasn't fully over it. She knew I recently got out of a bad relationship/engagement too, so she was upset and i told her I understood.
After a few months though, she became really snappy and distant. I called her to check up on how she was doing, and she snapped at me and I ended the call. After a week, I hadn't heard from her, not even an apology.
I decided to remove and block her from my social media and I didn't receive any more messages from her, so I deleted her number.
About 8 months later, I felt bad for how I acted and requested her and she immediately declined it, so I reblocked her.

It's now been a year since we went out. But the other day, she was at my work along with her sister and her parents. I came in and saw her, and I went to my area, and she and her sister followed me and just kept watching me. Even when I'd go to my work van outside, they'd follow me out there too.
But all they did was watch me, and they even got the attention of my AP.
I stayed in my work van until they left. What's weird is my location is 40 minutes from their house and they have one right nearby.
Why were they watching me?
Why was she following me around?
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