There are several things that it could be and I've been through pretty much all of them.  Acute abdominal pain is a major problem and one that many ER's/docs have issues with finding the real cause.  If you have to go to the ER repeatedly, do so!!  It is very important that you figure out what the cause is and by going to the ER a couple times, they understand this is not a tummy-ache, stomach virus or other less-significant problems.  They will then give you more tests and better procedures to identify the problem.  Here are a few things it could be:

1) Gallstones:  Gallstones usually cause pain in the middle to right-upper quadrant of your abdomen.  They are frequently triggered by high-fat content foods.  Pain can range from dull to sharp and stabbing.  This pain also usually comes and goes in waves. Most frequently diagnosed by an ultrasound.

2) Pancreatitis: This is usually an inflammation in the pancreas in presents very similarly to gallstones, but doesn't necessarily happen due to high-fat foods.  Also diagnosed by ultrasound, but also mri or cat scan.

3) Appendicitis: This is a sharp, stabbing pain in the lower right quadrant of your abdomen.  Both pancreatitis and appendicitis are very acute issues, so an ER might rule these out first.

4) Gastritis: This is the inflammation of the lining of the stomach.  This can happen before it becomes an ulcer.  This would be the most common diagnosis for stomach pain and the most mild.  The ER usually gives a GI cocktail to calm down the inflammation.  Now, several medications can cause gastritis.  For instance, all NSAIDS (celebrex, tylenol, alieve, indomethacin, etc.), pain meds, mixtures of medications, etc.  So even if they treat the gastritis, if you stay on these meds, then you will keep getting gastritis, and it will usually develop an ulcer if not resolved.  Sometimes it also helps to be on a proton-pump inhibitor, like pepcid, prilosec or zegrid.  GERD can be linked to gastritis an also feel just like it.

5) Gastroparesis: This is where your stomach slows down its churning process.  The stomach normally mashes all the food together with acid for digestion; however, sometimes the stomach muscle becomes weak or is slowed causing sharp stabbing pain in the middle of abdomen, especially after eating a large meal.  This can happen naturally (although it's abnormal), but it can be caused by medications as well.  Narcotics, in particular, are the most frequent medicinal cause of gastroparesis.  It usually takes several years of narcotic use before it happens, but not always.  

6) IBS, IBD or Chron's:  Irritable bowel syndrome is basically a sensitive digestive tract that responds acutely to stress, emotions, pain, etc.  It can cause bowel cramping, constipation, diarrhea and other problems.  Irritable bowel disease is usually more severe than IBS, but very similar.  Chron's disease is even more severe and is an auto-immune disorder.  Chron's is linked to AS because of the auto-immune origin.  Chron's is often seen in patient's with AS and can be treated with immuno-suppressant therapy.  Unfortunately, these are all subjective diseases, they can do a colonoscopy and look for inflammation, but it's a series of symptoms that helps to make the diagnosis.

There are other kinds of problems, but these are the ones that are most common and most familiar to me.  I've had my gall-bladder removed, dozens of gastritis episodes, GERD and have just recently been diagnosed with gastroparesis.  I can't take NSAIDs because they always cause gastritis, usually in a matter of weeks.  For whatever reason, my stomach is uber-sensitive to medications, especially those that mess with your stomach.  I've only been on a narcotic regimen for about 2 1/2 years and not even high doses, but I've developed gastroparesis that is KILLING me!  I'm nauseous and vomit daily, and don't get me started on the pain.  This has been going on acutely since November.  I've recently stopped all my narcotics to do a trial and see if they are the cause of my gastroparesis or not.  Now my pain is out of control :(

All I can say is make sure to work with your Rheumy and your primary care doctor and be sure to impress upon them how often and how bad this pain is happening.  It also helps to keep a journal of when you are having the pain, about how long, quality of pain and any triggers you can think of.  Again, this is often a tricky thing to diagnose, but I personally know how miserable it makes you, so try to be as helpful as you can to assist the doctors.  And when in doubt, go to the ER.  They misdiagnosed my gall-stones for gastritis 5 times before they figured it out and then I had to have emergency surgery!  It is very, very frustrating.  They will most likely send you home several times without answers.  All these diseases are primarily subjective and all present similarly.  It's one of the most frustrating problems for doctors, because they don't know.  If it continues with no answers, someone should refer to you a GI specialist, and if they don't, request one.

Best of Luck,

K

Kerry Monique....thank you for your very helpful post.

Thanks for all your input! I ended up not getting worse throughout the evening. But I will deffinetly keep an eye on it and speak to my Rheumy about it in may. I've been on enzymes for months now and it has helped immensley,thanks!

Definitely sounds like it could be gallstone related...

Maybe lack of enzymes to break down what you eat?

Many with AS have gut issues that need to be evaluated and treated. Do you also feel sick with this pain? You may need a CT scan, and the ER may be the best destination for you this weekend. During my first ER CT scan they could tell the ilieum was inflammed, possible crohn's. But since it had never happened before, they set me up with a consult with a GI doc the next week and sent me home on antibiotics. The next ER CT scan (16 months later) showed the same thing, so back to the GI doc who then begins to evaluate and treat for crohn's. It may be nothing, but I hope you get some answers sooner than I did.

Crohns?