I've found that in the effort to fit everyone into a simple diagnostic category, many doctors try to label someone with the most likely diagnosis-IBS. Have you considered a food related problem such as lactose intolerance or dairy or mild gluten intolerance?
Celiac is extreme gluten intolerance and only requires a blood test and it can also be observed when they perform the upper endoscopy. The blood test will be negative though if you have a non-severe form of gluten intolerance.
If you have a lot of gas/belching that may be the reason for the left sided pain (where the stomach resides). If belching and bloating, it may very well be some type of food intolerance.
Pancreatic cancer is rare and has been in the news lately which makes us all more paranoid. With pancreatitis or pancreatic cancer, I can tell you usually it is severe pain and radiates to the back. It is also often associated with a history of heavy drinking.
Also, with your change in IBS symptoms, nexium can cause diarrhea.
Just some thoughts, I hope it helps give you some ideas.