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!
When practicing vipassana, you simply just witness whatever sensations that are happening moment to moment. If a mental reaction arises as you focus on the belly, like a thought, an opinion or a feeling then simply note it and go back to the belly. If you can't  feel anything then note whatever reality is in your attention at that moment and if it seems like nothing then it's ok to note "nothing". Maybe you would be waiting for the breath to arise, if so you could note "waiting". You know, even if you get confused in your practice you can note "confused".

Truth itself is not dangerous but our reactions to truth can be dangerous. If you feel a bodily sensation isn't right then you might note in your head "doubt", "fear or whatever seems appropriate. Of course call a doctor if by "not right" you mean you might have something physically wrong. 

Getting a good teacher is one of the most fundamentally  beneficial things a meditator can do. If you have one be sure and ask them about any outside advice you get.