Hi Al39,
wow you just described me in my 20s. Mine started the same way. Pain in my lower "tail" area, the coccyx or whatever that lil bone on the end of your tailbone/spine is. And the right eye twitching.
The right eye twitching turned into what the doctors kept thinking was "pink eye" or some sort of eye infection for the next 10 years. It was actually uveititis/iritis(sp?) sorry bad flare morning

Late 30s to early 40's is when most men get dx'd too, not sure what the stats are on us women not many that I have found. Not sure if you are male or fem by your screen name so sorry

but you fit the age range.
I do have full blown AS diagnosis after 20 years of thinking I was nuts and a hypochondriac. Has your doctor done a blood test on you for the HLA-B27 gene marker? That is the real test.
tightness in you ribcage is also a known symptom, the acid reflux could be as you said the anti-inflammatory pills or just plain old acid reflux.
Some of the other early symptoms that I had were fatigue, a general feeling of flu like symptoms, body aches and the iritis. The back of my eye would swell from the inside due to the body thought there was an infection or my eye was the enemy or something and try to kill it. Hurts like heck. It wasn't until my hands had swollen up like balloons one day before the docs thought hey this might be an auto-immune thing. I had a sed rate out of this world but was serum negative for RA.
You are very lucky to actually have found a doctor who is not only aware of AS but is willing to do more tests. Even if he did say it is unlikely I would go back and ask if they did the HLA-B27 blood test and if not could they just to put your mind at ease and rule out what could be there.
Or talk to your regular doc tell him/her what the ER doctor said and request it. Ask family members if any of them had any weird type "Arthritis" and if it runs in the family. AS usually does. After my Diagnosis I had fam contact me saying "you know Aunt so&so had some weird form of arthritis and so did Aunt so&so both on dad's side. About 4 of them popped out of the woodwork. So check with older family members. If so this strengthens your request for the blood work to be done.
I hope you don't have it. This is not fun I would share with someone I hated. Hope you are lucky and it is just bad posture and the need for some corrections on that and some other form of back related exercises.
Funny I am not on here much but popped on today bcuz I woke up feeling like he!! and said hmmm let me see whats on KickAS
Hope to see a post from you real soon saying NO AS!!!!:)
good luck hun
Christina