Hi Nancy,
Welcome to our internet family. Please don't feel alone in your impression that this is an overwhelming disease. The medical community tends to treat it as a nuisance disease, since it is rarely fatal, but the fact is, MC is as life-altering as any of the other IBDs, and unfortunately GI specialists know far less about treating it than they do the other IBDs. That's why this discussion and support board exists.
Nancy wrote:1. Is there any chance I don't have LC? (long shot, I know)
Well, the probability of a misdiagnosis is not zero, but it is so low as to be zero for all practical purposes. I have never heard of a case where MC was diagnosed when it was not present. GI docs (and pathologists) very frequently fail to diagnose MC when it does exist (they mistakenly call it IBS, especially if the GI doc fails to take enough biopsy samples from all segments of the colon), but they virtually never diagnose it when it does not actually exist, because that would require an almost impossible level of incompetence.
Nancy wrote:2. My medical history includes anxiety, depression, GERD, esophageal spasm, canker sores, and fibromyalgia. Probably all related, right?
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Definitely. Those associations are very common, and most of us find that as we bring the inflammation that causes our MC under control, our symptoms associated with those other issues usually either significantly improve, or disappear completely.
Nancy wrote:3. My biggest question- if I begin an elimination diet now in an attempt to figure out what might be causing all this, how will I know whether the results are related to the medication or the food? Should I try both of these at the same time? I feel like maybe I need to get these symptoms under control first, before taking that on.
For many patients (though not all) Entocort EC (budesonide), can mask the symptoms of MC. It takes a while for the diet changes to bring improvement, because the damage caused by the extensive inflammation is slow to heal. So many members here use Entocort while they are getting used to their diet changes, and allowing their gut to heal. But the time when you should be making your diet changes is early on, because if you do not eliminate all of the foods that are causing the inflammation, then as you begin to reduce the dosage of Entocort (or in some cases, few days to a few weeks after the drug is discontinued, depending on how fast you wean off of it), your symptoms will all return, and you will be back to square one. Drugs can mask the symptoms, and reduce the inflammation, but they cannot prevent the inflammation form being regenerated. Therefore, when the medication regimen is ended, if the inflammatory foods are still in the diet, the symptoms will soon return. So if you believe that the medication is sufficient for remission, that's easy to verify by simply not changing your diet. Most patients who trust the drugs to bring lasting remission though, end up very, very disappointed.
There is one exception to the need to change your diet. In some cases where the MC is drug-induced (and many cases are), if it's caught and corrected in time, food sensitivities may not develop. IOW, in a few cases of drug-induced MC simply discontinuing the use of the drug (or drugs) that is/are causing the inflammation can bring lasting remission, without further intervention.
Based on your list of autoimmune type issues, I'm going to guess that you are probably taking more than one drug that is frequently associated with causing MC, and in that situation, the odds of not having food sensitivities are rather low. Drugs that often cause MC include (but are not limited to) antibiotics, NSAIDs, PPIs, SSRIs, SNRIs, statins, bisphosphonates, beta blockers, and based on the experiences of a number of members here, it appears that the anti-TNF drugs may also either cause or perpetuate MC. In some cases, H2 blockers can be a problem, but this seems to be a low-risk item for most patients.
To further complicate the issue, if you are taking any of those drugs, and they are indeed associated with the inflammation that is causing your MC, then it will almost surely be necessary to discontinue the use of those drugs in order to control the inflammation. IOW, just changing your diet will not be sufficient, if you have drug-induced MC and you continue to use any of the drugs that are implicated in your particular case.
Incidentally, the serum tests used by doctors to rule out celiac disease/gluten sensitivity are worthless for detecting gluten sensitivity in most cases. Most of us here are extremely sensitive to gluten, but we always test negative to the classic celiac blood tests. Mainstream doctors have no way to officially diagnose gluten sensitivity, because they refuse to accept the validity of the stool tests used by EnteroLab to detect food sensitivities (even though those tests are several orders of magnitude more sensitive than the blood tests, and they are very reliable).
Using the current diagnostic criteria for celiac disease, the extent of villus damage in the small intestine has to reach at least a Marsh 3 level before most GI docs will acknowledge celiac disease/gluten sensitivity. Using the standard celiac tests, the average length of time from first symptoms to an official diagnosis of celiac disease is still 9.7 years. That's a long time to suffer simply because the GI docs don't know how to diagnose gluten sensitivity before the damage to the gut becomes extensive. Most GI docs don't even realize that MC also affects the small intestine. In fact, it can (and often does) affect and inflame any and all parts of the GI tract.
Again, welcome aboard, and please feel free to ask anything.
Tex (Wayne)
Truthfully, I feel best when I don't eat at all. 
