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Well, I can very well understand your concerns. It seems that you probably have one of the two potential conditions.
Considering your age and chronicity of the symptoms with no apparent weight loss or bleeding or reduced appetite (I assume since you did not mention), does not seem to be a severe thing.
You probably have peptic ulcer disease, the cause of which can be an Helicobacter pylori infection. Helicobacter pylori's sensitivity of testing stools goes down if someone is taking Omeprazole while giving the stool sample. The test can come false negative, whereby the bug is present, but it does not show up in the stools due to the suppressant effect of Omeprazole type medications. Did you get this test while you were on Omeprazole?
The other possibility is Gastroesophageal reflux disease (GERD), which is indeed a common condition for anybody. And which drastically responds to Omeprazole type of medications.
Now I think, since your symptoms are rapidly controlled with Omeprazole, I suggest you continue with that.
Endoscopy was an excellent option to proceed with alarming symptoms such as weight loss, positive family history of early stomach cancer, bleeding, or decreased appetite. Do you have any such signs?
Caffeine is a commonly known precipitant factor for GERD. And therefore, this was probably worsening your symptoms as well.
Let me know some of the questions I have asked above; I would guide you when you should test H. pylori and whether it is required. Similarly, I would be able to tell you whether you need endoscopy or not. Consult a doctor to treat it soon.