I hope I can give you some guidance here.
Generally, people with stomach ulcers tend to feel better after eating; this is because the food helps neutralizes the acid thats in the stomach. The pain returns when the stomach is empty.
Pain AFTER eating as you describe is more commonly associated with heartburn or GERD, a situation where stomach acid refluxes up the esophagus.
However, you have been on celebrex for quite a while, and even though its gentler than its chemical cousin ibuprophen, (motrin), it to is implicated in ulcers. And not all people show the textbook symptoms for disease, so an ulcer certainly cant be ruled out.
Neither ulcers nor heartburn is associated with normal diarrhea. I am happy you said your diarrhea was not black and tarry, as black tarry stools can indicate the presence of blood.
I havent seen you of course, so my answer is only my best guess based on what youve written. Your diarrhea may possibly be from the Septra you have been taking for so long. Septra, as all antibiotics, dont discriminate and will kill the normal bacteria and the bad. So if the balance of good bacteria is off in your gut, you CAN have diarrhea. C diff can flourish as a result, but you tested negative for that. Often when C diff is tested for they will run tests for shigella as well, Im assuming that to was negative.
Your stomach aches may be from the spasming of the intestines which contribute to diarrhea.
Another possible process that could be occuring is ulcerative collitis, again most likely caused by the Septra. This is inflammation of the intestines, and it to can lead to diarrhea.
I read also of the joint and muscle pain, and of the history of fibromyalgia and osterarthritis. Its possible the increase in pain may be due to a flare up of the fibromyalgia; its also possible the pain has increased because you stopped the celebrex.
There are some things you can do, in consultation with your physician. Replacing the beneficial bacteria may be an option, and that can be done with supplements of lactobacilis and acidolphilis. These are available in health food stores. Yoghurt also has this bacteria but not in sufficient numbers.
You can also ask your doctor for a medication to help with the presumed intestinal spasms...if this is not ulcerative collitis.
The endoscopy can tell whats happening in your esophagus, and see if there is an ulcer, but I would also reccomend a lower GI evaluation. That would help to identify a collitis as well.
Here is information on heartburn: http://www.mayoclinic.com/health/heartburn-gerd/DS00095
Here is information on ulcers: http://www.mayoclinic.com/health/peptic-ulcer/DS00242
Here is information on ulcerative collitis: http://www.mayoclinic.com/health/ulcerative-colitis/DS00598
I hope I was able to help,