Is it normal that i convulse when i ejaculate?

I started masturbating when I was 12 bet ever since I was like 18 I go through these weird convulsions when I cum. I don’t know if it has anything to do with it but I shoot way over my head and my mother couldn’t understand what the stains on the wall were. Then my dad came to me and told me to use Kleenex when I jerk off. But now I live alone and just it fly. It feels better but I don’t understand the convulsions.