Hello. It is possible that you may be having Giardiasis or amoebiosis because of eating at hotels which may be having poor hygiene. The Giardiasis is common and recurrent food and water borne infection in poor hygiene settings, giving rise to such symptoms you are having.
you were seen by a gastroenterologist and he gave you an antibiotic and for nausea. most GIT infection wont show on endoscopy or in US.
If your dr suspected Ulcer he could have done urea breath test for H Pylori infection. This is non invaisive and reliable test to exclude H Pylori infection , the cause of Peptic ( gastric ulcer).
Did you have any food alergies , like Gluten allergy.?
Next Steps
Advice.
Get your stool tested for ova/ cyst from a good lab.
If the test comes positive the treatment is easy.
or you may be having irritable bowel syndrome( IBS)
get Urea breath test done.
Health Tips
try to have food at home if you can.
drink bottled water at hotel
wash your hand well before hsving food
avoid salads and raw vegies at hotel.
After the tests you can write me back.
if you will be diagnosed with IBS ,its not curable but you can avoid the symptoms by keeping some food habits.