No matter what I eat or how much I eat I get nauseated. I had gastric bypass surgery 18 months ago. What could be causing my nausea?

Gallstones or ulcer. Nausea after gastric bypass is not normal. Most common is small stones or sludge in gallbladder. Also consider ulcer (must avoid aspirins and tobacco). Less common might include bowel obstruction, medication-related, and constipation. Of course, you will need to see your doctor about this. Good luck.
You need more workup. Worst case scenario you may have internal hernia, but more likely you have narrowing at the anastomosis. I recommend to start from upper gastrointestinal xray followed by endoscopy and possible ct scan. If you from la you can come to see me for appointment. Call 310 623 1786.
Ulcer or gallbladder. Most common causes are gallbladder or an ulcer at the connection of your pouch to intestine. Other causes could be an internal hernia. Whatever the cause this is not the norm and you should follow up with your surgeon.