Guidance for how to handle stomach problems arising during Vipassana?

During concentration the main thing I can feel during the first half hour or so is the feeling of the stomach/ top part of the intestine area or feelings inside the belly. There's really nothing else for me to feel, especially when mediating topless so that the skin's interaction with fabric doesn't give a sense of rising and falling. Although I'm aware painful sensations can be expected, this uncomfortable feeling does feel like the wrong type of discomfort but is my only point of focus. It seems like something I shouldn't be focusing on, or maybe I am over extending when breathing, but I don't think so. That being said, my practice has eventually led to pleasant sensations and having a tension to wave-like movement when breathing. Anyone have any guidance or suggestions? NB my posture is quite straight. Thanks!