In “bellyache,” Billie tells a fictional story about murder from the perspective of a psychopath. The song focuses on themes of guilt, as revealed by Billie in an interview with Vice:

‘Bellyache’ is about the concept of guilt, when you do things in the moment because you feel so strongly about them. In the end you’re left with the decision you made. That line—"I thought that I’d feel better, but now I gotta bellyache"—is about how you kinda know that you’re the worst but you don’t care. It’s about a psychopath who regrets being a psychopath but doesn’t really care.

Billie later revealed that the main inspiration for the song was Tyler the Creator’s “Garbage:”

Garbage was one of the biggest inspirations for writing my song, “bellyache.” It’s more graphic than my song, it’s more literal and “I’m killing people and putting them in my basement” kind of song.…I’m a huge fan of Tyler, he’s been my biggest inspiration ever.