Hi,
Welcome to the ehealthforum and I am glad to help you out. There are 2-3 likely possibilities in your case.
The first is a gastric ulcer/gastritis. This can result in a pain which comes immediately after eating.
The remedy is antacids & drugs which decrease the acid production like omeprazole, rabeprazole etc.
The 2nd possibility is the gall bladder disease (gallstones). This can also present with pain and nausea immediately after eating but in this usually the pain is more in the right upper abdomen which may radiate to the shoulder.
An USG of the abdomen helps in the diagnosis. The 3rd possibility is acute pancreatitis.
This is diagnosed by doing a serum amylase/lipase which are raised and a CT scan of the abdomen is the confirmatory test. Please do consult a gastroenterologist for the confirmation of the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.
For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.