In the original run of the Sailor Moon anime, Usagi will occasionally say the hard to understand (and even harder to translate) line about her stomach playing a trumpet:
Japanese: 「おなかのラッパがプー」 (onaka no rappa ga pu~)
Literal Translation: “My stomach’s playing the trumpet”
Localization: “I’m laughing so hard, my tummy hurts!” / “My tummy’s singing!” / Etc.
On the surface of it, and depending on context, it sounds like she’s saying that her stomach’s rumbling, but as you watch through the series, you realize that she uses it more like a personal catch-phrase and not with any one, specific meaning. For example, this phrase comes out both when she’s yelling at Mamoru1 and again later when she’s on her way to lunch and can’t wait to start eating.2
So what is this, just another Japanese idiom that doesn’t actually translate well into English? Actually, the story behind it is more interesting than that. It turns out that it wasn’t originally part of the script and was said to be an ad lib on the part of Usagi’s voice actress, Kotono Mitsuishi, in the middle of Usagi’s tirade against Mamoru shown in the image above. But that isn’t where this catch phrase actually really originated.
Actually, this phrase first came about in the manga/anime known as Goldfish Warning!3 and was often uttered by the overly active Wapiko.
After the anime ended, most of the animation staff (including director Junichi Sato and music composer Takanori Arisawa) moved over to the the Sailor Moon anime, which led to the appearance of many references to this series throughout the first season of Sailor Moon. From thereon, the phrase took on a life of its own, and has become one of the many Usagi-isms that continue to appear throughout the entire run of the anime.
Though this doesn’t really explain what your stomach trumpeting has to do with being emotional or agitated, it’s nice to know the story behind it!