You can find good stomack medicines at the "farmacias" : they have PeptoBismal and other remedies, even Kaopectate...Do not take antibiotics, just lots of bottled water, Gatorade, and other liquids...
Be careful of what you eat!! Nothing on the street, and nothing you don't peel yourself.
If you can, go see Carlitos at Dom Ham, over near the Centro Medico Canela, he can help you with anything to see or do in La Romana. "Dom Ham" is THE place for fun folks of a certain standard. No cueros or low life..
HB