Hi,
Welcome to health care magic,
The causes of gastritis are helicobactor pylori infection,oily and spicy diet,or drug induced.
In your case,stomach pain with nausea could be because of dietary or infection but needs to be evaluated further by various investigations.
You can go for routine blood count,ESR,
urinalysis with ultrasound abdomen with upper GI endoscopy to rule out other gastric pathology and to confirm
gastric inflammation.
You can continue h2 blockers like ranitidine or
proton pump inhibitors likepantoprazole,
rabeprazole etc with cytoprotective drugs like sucralfate,
misoprostol etc.
It underlying cause is H pylori then you will require triple drug regimen means two antibiotics and PPI.
Consult your gastroenterologist for further advice