Not sure why this dream happened

Date: 12/10/2018

Well. This is a part bc I dont remember all but I dreamt I was at some sort of school or big lodge...? Anyway. I'm just sitting at a table and it is winter so it is cold. I'm bundled in a coat. My ex gf from years ago walks in with her husband. I dont speak to her I just think oh didn't expect her. Everyone was thinking it isnt coincidence and we showed up to rekindle something. It flashes forward into this announcement where people are being tricked into going to the library next door with their significant other or crush. Well...now I'm walking outside where two drug addict girls are standing. One of them is choking or something. Blood/mucus is coming from her nose and mouth. I turn around to go back inside to tell someone. We come back out and she is dead. Then a woman walks out and hands me a postcard type letter that says go to library. Ok? So I proceed. I'm almost there. There is a picnic table outside the library where 1 or 2 girls and like 4 or 5 guys are. One pulls a gun, aims and bam. But he shoots behind me. My ex is laying in the snow dead about 25 ft away. I run into the library where a guard is and I say what happened. He tells me go up the stairs, 2nd story, 3rd door on the left. That is where police are stationed waiting for reports. So I go. On the way there, I'm stopped by a girl who is laughing and she offers me the gun they killed ex with. I run and as I'm just at the door, the guy who shot stabs me in the chest out of nowhere. Then I wake up.