@codyprimmer- Hello and I'm so glad that you decided to tell your story. It's so frustrating to not know what is wrong with us when something unexpectedly shows up. I think that getting a GI workup done is a great idea. I couldn't believe the pain I felt on the way to flying to the opposite coast a few years back. I already had SOB because of my lung cancer but not stomach pains. I called my husband as soon as I landed. He had GERD and knew that's just what I had by my symptoms.
Pain in the stomach is hard to diagnose, as you have seen here. There are a multitude of symptoms and I believe that there is always a cause. It very well could be your posture (I sit too much too for the same reason as you). Or it could be a combination of things.
I would consider getting an appointment with a GI doctor soon because your running could be making it worse, or not. Best to get it checked out.
Here is some more information for you: https://www.webmd.com/pain-management/guide/abdominal-pain-causes-treatments#1
Do you know of a good GI doctor?