Someone threw something at me

I was sitting in a hallway in my college waiting for my next class and 3 random dudes came past and threw a piece of paper at me. It landed on my head and then slipped on the floor. They didn’t even laugh or anything, just walked past after throwing it

Why can I never stand up for myself in these situations? I’m mortified I was so shocked I just sat there. I wish it wasn’t so obvious I have anxiety and maybe this wouldn’t happen