I get the same stomach ache but it does not last as long as your has, I also went to a GI had the upper endoscopy, yep large hiatal herina, sent home with medicine to help the food pass thru my stomach.
I started having this pain back in 2001, I then had surgery for removal of a 3 lb fibroid tumor, the stomach pain went away, 2003 pain back again, then I had surgery for a partial hysterectomy [ had another fibroid] a hernia repair [from the 1st surgery incision] and a tummy tuck!!! the stomach pain went away. Now, I get the pain and I have been associating it with foods like cheese, and milk, mostly dairy products give me that pain, so I have stopped eating those foods
So notice what you are eating, maybe you can associate that pain with a specific food? Maybe your primary care dr can help you without going back to the GI. I don't know anything about stomach cancer, but don't stress over something you may not even have and maybe thinking you have that is causing stress to cause the stomach pain??
Be happy don't worry