Songfacts®:

• In this song, Billie Eilish has a bellyache because she feels a little guilty (just a little) about murdering her friends and stashing their bodies in the back of her car. It's not was most 15-year-olds would write about, but Eilish was no ordinary 15-year-old. Even then, she was writing unusual and often macabre lyrics that were sometimes personal, but often in the guise of a character, in this case a serial killer. Eilish made it clear she was just having some fun in this song and harbors no bloodlust.
• Eilish wrote "Bellyache" with her brother-producer Finneas. It was released as a single in early 2017 ahead of her first EP, Don't Smile At Me. By this time, she already had a record deal with Interscope thanks to her 2015 song "Ocean Eyes," which went viral in 2015. The EP kept her snowball rolling downhill, and by the time she released her first album, When We All Fall Asleep, Where Do We Go?, in 2019, she was more refined and well versed on how to handle the media.
• Eilish made a video for this song that was directed by the Los Angeles team Miles and AJ. We see her on an empty road in the desert, wearing a yellow jumpsuit and pulling a wagon with two garbage bags as cargo. She seems a little unhinged, at one point pulling out a wad of cash and making it rain. At the end, when she encounters a cop, there's a standoff, which is how the video ends.
• A few months after this song was released in 2017, it was remixed by Jeremy Lloyd of the duo Marian Hill, who had a breakout hit that year with "Down." Lloyd and Finneas are friends and have similar sensibilities when it comes to production. In a Songfacts interview, Lloyd explained how he put it together. "That was one of the easiest and most fun remixes I've done," he said. "I loved the bassline that came in on the chorus of the original, and I wanted the whole song to be like that.
  
  I took the bassline that they had in the chorus and I put it over the whole song with the vocal a cappella and then built my version over that. Sometimes I get a remix and I'm like, 'Hmm, how will I reimagine this?' and every now and then I know exactly what I want, and that was the case with this one."
• After Billie Eilish got to know Doja Cat's music through SoundCloud, she asked the "Say So" hitmaker if she wanted to do a verse on "Bellyache." Doja loved the demo she received, but hit a creative logjam. "I remember thinking it was so cute. I loved it. I just couldn't think of anything to write," she recalled to Rolling Stone. "It was one of my writer's-block moments... And I remember seeing that song blow up and thinking, 'Good for her. That's awesome.' I don't think the song was for me, though. It was quite hard to write to."