I have had bad bowel and stomach problems for at least 30 years, Since Feb. I've lost 80 pounds. (I was obese, now I'm not.) I had to be on a "clear liquid" diet for almost 2 months. I had an EGD/Colonoscopy, many blood tests, 2 cat scans with contrast, and spent a week in the hospital just last month. As a result, we discovered that I am profoundly "intolerant" to all Proton Pump Inhibitors -- Prilosec, Prevacid, Nexium, etc. that I have been taking for the past few years, and kept getting sicker and sicker. My Gastroenterologist said he's never seen anyone who is so intolerant to a whole class of medications like that. My bowel is still extremely inflamed, and my diet is very restricted. BUT I THINK I AM FINALLY ON THE ROAD TO RECOVERY! Stopping the PPI's helped a lot, almost immediately. I can't take any pain meds. They make me much sicker and cause awful nausea and vomiting. When in the hospital, I was vomiting constantly, and they couldn't stop it with the very expensive Zofran, that helps most people. We found that plain old Promethazine stops my nausea immediately. I am taking Librax for the IBS, Levsin for the extremely painful abdominal cramping (and it helps with my slow-emptying stomach), Promethazine for nausea, and occasionally Pepcid or Zantac for any acid stomach, because they're not PPI's.
My message to you is to: go through the GI testing -- it's critical to rule in/rule out other diseases. Keep after your doctors to LISTEN TO YOU. Keep a symptom/food/medication diary, as others have suggested. That too, is critical. One of my problems is that I tend to try to eat more normal foods after a few days without symptoms. I have learned to go slowly. Introduce more normal foods slowly and only one at a time. Then keep track of your symptoms. My GP pooh-poohed my requests for a referral to a nutritionist, but when I was in the hospital last month, I just told one of the nurses that I wanted to see a nutritionist, and one came up the same day. I have a specialized diet sheet that I refer to now, and I can call the nutritionist whenever I have a question.
For me it has been best to avoid all over the counter drugs. I talk to my GI specialist before I make any med changes. It's not easy to get these docs to see you in a timely manner ( I usually have 6-8 week waiting periods before I can get in to see my specialist.) My GP is learing about the management of severe IBS, and that has caused lots of problems, but she's learning, just like I am.
Sometimes it helps to have an advocate assist you when you deal with docs. That can be a family member, a friend, or if your relationship with your doc gets really bad, your insurance company can sometimes help -- I called "customer service" because my specialist hadn't returned my calls after I had left phone messages (during the work week) to his nurse for 3 days in a row that I was in agony and unable to take any food at all, and no one would call me back. An hour later I got a phone call back from my doc. I wouldn't recommend doing that unless you're really out of options in your area, but as a "last ditch" effort, it certainly does get the attention of the doc's support staff and the doc.
You have to keep accurate records of what's happening to you, inform your doc, get the appropriate diagnostic tests, and remember that it's going to take time and patience before you see improvement, then build slowly and carefully on what works for you.