You know, it does seem like the only thing docs want to do is test, test, test, and nobody puts on their thinking caps and nobody tells you anything. I sure do feel for you and your wife. I don't know what bloodwork they did, but if you still have that paper, you might want to study it, and I suppose they should have looked for signs of infection, or for anemia, or something like that. By the way, even tho the scan exposed her to some radiation, it was good to find out nothing obvious is going on in her body! But next time, you can ask for an ultrasound instead, no radiation, and it shows a lot of stuff too.
A colonoscopy does not have any radiation. They sedate a patient to where they feel relaxed and sleepy, and they examine the large intestine with a tiny scope, and it also has a mechanism on it that can "take samples" of mucous in the bowel, to see if she's got an infection going on that needs treating. It also looks for precancerous bumps and painlessly removes them during the exam. And it can see if there's a blockage of some kind and what the source might be, like a twisted bowel or whatever. Before she sees that gastro guy, try to have her scan pictures and report forwarded to his offices. It's always good to have another set of eyes on those, particularly a specialist.
In the meantime, I would like to suggest there's a possibility your wife has some gas in her guts, which can cause an awful amount of pain. I mean, this is the simple solution, but it's possible. Could be she's had bouts of it, which might explain why it's been going on for so long. Alka seltzer, once a day for several days, might give her some relief. She may also be a little constipated, either from medicine or a lot of heavy food, so in order to break down waste faster, another simple thing to try is for her to drink Acidophilus milk for a while or eat Activia yogurt, both available at the grocery store. Also to help any sort of constipation is these three: extra water, extra exercise, and extra fiber (oat cereal, wheat bread, salads).
See if she had a scan of both her abdomen AND pelvis. If not, her gynecologist can do that ultrasound I mentioned of her pelvis and can also do a repeat urinalysis, to make sure her kidneys are okay...kidney stones can hurt a lot. And lastly, if her pain is in the very lower right part of her abdomen, where the appendix is, then she could have an appendicitis...I hesitated to mention it, because it's the obvious thing to look for first. If that's where the pain is, she may need to get that removed soon.
Keep an eye on your wife's temperature. Put some heat on her abdomen, either with a hot water bottle or a small electric heating pad, because heat helps pain. Very warm baths are relaxing, which she needs to try to get comfortable. I hope some of this helps.