Welcome to the forum. You need to visit an emergency room. Do not stay back at home. I think you need to have an urgent ultra sound done to rule out gall bladder
stones. The pain on the right side can be due to enlarged liver and the other symptoms like fever, loss of appetite, pain and vomiting can be due to gall stones. First help your doctor to rule out gall bladder stones. You need IV fluids, antibiotics, analgesic, antacids and dietary supplements.
Since you are already diagnosed with duodenal ulcer
you need to be clinically examined and investigated ( routine blood and stool test ) to get proper management for your ulcer. You should continue the pantoprazole
and diet menu suggested by your doctor.
Do not get worried USG will help to rule out inflammed appendix. You need to have a few blood tests also. You may require immediate surgery depending upon your USG reports for appendix and gall stones. If you have gastroenteritis
you need not worry just get admitted and get chemotherapy
done. You will be fine. There is nothing to be scared of just co-operate with your doctor.
Have a good day. Take care.