It sounds very much like pancreatitis.
Does leaning forward, when sitting, help relieve the pain?
That is one clue. Labs for Amylase and Lipase levels would be done.

I had chronic pancreatitis. I don't drink alcohol so it was from something else. My pancreatic duct was blocked, possibly from extremely high levels of triglycerides for a long time, but there are other causes, including the herpes simplex virus, which is what I suspect. During an attack, I was advised to not eat because eating stimulates the pancreas.

Years ago when I had it, I would have vomiting. I was always wondering how I could have the flu when no one around me at work or out of work had it.

I had many episodes, sometimes just the unrelenting pain.
The last time I had sudden nausea and the pain from upper abdomen through to the back. After the pain lessened, I would have a dull achy pain in my upper abdomen, and when I ate food, it felt like I was trying to digest rocks. I was tired all the time, slept a lot, lost a lot of weight. After a year of tests and scopes to try to open the duct, I had part of my pancreas removed because that duct was so scarred it was not going to open, and if the pancreatic enzymes could not be sent out of the pancreas, then they would eventually destroy the pancreas.

So, anyway, that was me. I hope you have a simpler solution

Don't let the gastro doc tell you it is just GERD or Acid Reflux if you think it is more.
I was lucky to have an aggressive gastro physician's assistant on my 2nd or 3rd visit, who ordered the CT scan because of the pain. Because I was never hospitalized for it, I think the gastro doc didn't consider it. I think I could have been many times if I had gone to the emergency room or if I would have known to see a doctor during an attack. But I didn't, for many reasons.

Here is a link if you would like to know more:

http://www.mayoclinic.com/health/pancreatitis/DS00371



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