Why am I so parnoid?

I used to be friends a girl but she would get mad at us for things that weren't our fault or a big deal. Me and a friend went to recess instead of the library(i was in middle school last year). Tell her this. Repeat it to her. And she still got mad at us. I can go on and on. That and her excluding one of us a year we stop being friends with her. We felt bad she had no friends but cursing at us and her telling each one of us that she loved us to get us to go back with her(not I love my friend love you as in I’m gay for you) she also texted about suicide and cutting among other things which made me feel terrible and scared I was 13 I didn't know how to deal with this. I still don't. It didn't really help when her mom called me screaming at me that I was bullying her daughter. This year I mostly got over her as she slowly stops messaging me she is in this discord chat and she talks bad about me even on times when I was the last person on it. But, I was walking the halls with a friend when all of a sudden I see that girl(ex-friend) she is taking a video of me as I'm walking down the hall. I had noticed this a few timesI ignored it but walked farther away. Me and my friend who doesn't even know this girl are the only ones in the hallway. Sure I have a witness but this girl isn't the most trustworthy( in teachers opinions) and what are they going to do. My mental health went down the drain after that. I feel like she is always watching or I fear she would do something drastic.