Brief Answer:
lot of potential causes

Detailed Answer:
Hello,

there are a lot of potential causes. I'll mention some common ones and how to proceed - according to my opinion - with the necessary investigation.

- Peptic ulcer: a history of peptic ulcer is usually present with burning pain either right after or half an hour after a meal, sometimes pain that awakens the patient at night with an urge to eat something, etc. An upper gastrointestinal endoscopy would clarify this issue.
- gallbladder disease: the pain is usually located to the right under the rib cage but central pain occurs sometimes. Clinical examination, an ultrasound scan and blood chemistries would confirm (or exclude) it.
- pancreatitis: severe central pain with lot of vomiting. Sometimes a history of gallstones. The pain is too severe to stay at home. A CT scan of the abdomen and an elevated amylase confirms the diagnosis. The ultrasound scan might be positive too.
- heart disease: although heart related pain is located on the chest, upper abdominal pain cannot be excluded even as the sole location of involvement. Nausea, vomiting, cold sweats are common symptoms. An ECG and blood tests (troponin mostly) would be very useful.

Evaluating all the symptoms you've mentioned I believe that the best way of action is to find a doctor for clinical examination and proceed with testing with ECG, ultrasound scan of the upper abdomen and blood tests which should include at least a complete blood count, C-reactive protein, urea, creatinine, amylase, troponin, liver function tests, glucose and electrolytes. The clinical examination data are absolutely essential. Absent bowel sounds for example point toward a serious event. A rigid and painful abdomen might be a sign of peritonitis... etc...

I hope you find my comments helpful!
You can contact me again, if you'd like any clarification or further information.

Kind Regards!

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