What did the shot in my ass do for my mono and will it make me feel better?

You’ll feel better: Lots of different “shots” but you probably got a corticosteroid (pills would have been equally effective). A brief course of steroids (either orally or as a “shot”) will help you feel better faster. Symptoms like fever, sore throat and/or swollen lymph nodes get better faster with steroids, although you’d get better eventually anyway - just slower