Why do I stutter

Why do I stutter I’m 18 M and I started stuttering about 3 months ago randomly. It’s mainly when I know what I want to say and wait until I can say it then I can’t even say it at all or just the first part of the word. I have been kicked in the head when I was 15 but no damage. It’s the worst in school when I’m up to talk about a subject I just act like I forget or don’t know when I know. I just hate everything