You need to see a GI. Scopes would be able to tell (biopsies specifically). I have Crohn's and gastritis. The inflammation in my stomach is just from NSAIDs. I always have mild inflammation found in my stomach, but the alternative would be giving up NSAIDs which I can't manage. So I deal with the stomach inflammation with a PPI and Maalox/Pepcid. Sometimes Carafate.
I have Crohn's in my colon and the last part of my small bowel. Biopsies showed very specific features of Crohn's which confirmed the diagnosis. Honestly, for me, the Crohn's has been much easier than the AS to manage.
I've tried about 12 NSAIDs. Mobic, Celebrex and Relafen all helped me. All three are easier on the gut than Voltaren or Naproxen. One that's hard on the gut is Indocin, I would avoid that one. I tried it several times and lasted only about 3 days on it. It is very effective for AS, but it was just too hard for me to tolerate.
Celebrex has become very popular and is generally considered to be the easiest on the gut.