As DianeB says, all good advice.
I developed gastritis after taking an NSAID for a while after surgery. Even while in the hospital following the surgery, a nurse, dietician, and the surgeon all told me at different times to eat lots of yogurt and take a good probiotic (Sherrine mentions probiotics here). They can really make a difference as to how your stomach feels, but of course, diet in general is important too. And as others mention here, food sensitivities or allergies are not at all uncommon. Sometimes it pays to be a food 'detective.'
I have a book written in 1992 by DeLamar Gibbons, MD called "The Self-Help Way To Treat Colitis And Other IBS Conditions" and it was very helpful to me, when I developed stomach issues. It's likely still available, if you do an online search. Very easy advice to follow.
Would you believe I developed chronic diarrhea in 1994, and it wasn't until 2011, when I had an upper endoscopy and also a colonoscopy (yes--worth it!) before a doctor would finally tell me it's how my system now functions, for some reason? I thought I had a horrible disease for a time, because nothing was really checked out.
I highly recommend those two tests. The prep is the worst, I think, and they basically walk you through the tests (I practically slept!). If your doctor recommends it, it's probably a good idea and might help pinpoint what exactly is going on, and alleviate any anxiety. Best wishes and keep us posted.
BTW, I love your name--so cheerful sounding---and welcome!!