a Fever
A fever is when your body turns into a monster and it wants to eat your brain. It's loud, it's mad, and it's going to ruin your day.
"My fever is like a monster and it’s eating my brain.", @MonsterFever
"I have a fever and I feel like I'm going to be eaten by a monster.", @MonsterEater2022
"My fever is so bad it's like a monster is inside my head.", @HeadMonster101