feeling nauseous constantly

As both of your doctors have suggested, a stomach ulcer or acid reflux are the conditions that can result in chronic nausea.
Acctually, some acid reflux sufferers instead experiencing chronic heartburn, have constant nausea as dominant symptom.
Testing for H.pylori is important, since its presence in gut is linked with stomach inflammation and ulcers.
Prevacid is medication usually used for controlling symptoms of acid reflux, and stomach ulcers.
When acid reflux symptoms aren't responsible to the given dose of Prevacid, the dose is increased and is dosed twice daily instead of the typical once a day dosing.
Also, it is important to get control over night time acid reflux symptoms that tend to be worse.
For that purpose, another type of drugs, that same as Prevacid suppress stomach acid, are suggested.
Taking a H2 blocker before bedtime has been shown to help with symptoms, since it acts faster and stops acid production at its early stage.
Lifestyle modifications are of the same effectiveness as drugs.

Best wishes!