Brief Answer:
The physical and the mental

Detailed Answer:
Congratulations on coming off the most dangerous narcotic (proportion of those with fatal overdose versus total on it is highest for methadone Prescribed for Pain; addiction centers treat younger, healthier people and watch them DAILY... and they die from heroin not the methadone!).
We're thinking it's the methadone. and you should first make sure about that with a standard simple physical and routine blood tests.
Liver/lung/kidney trouble. weight loss with low blood pressure. Stomach irritation with or without bleeding and/or anemia. would be pretty obvious things that could be the whole issue.
back to the methadone
While the textbooks don't say much about it, my addict friends assure me there is a late stage withdrawal from methadone. Muscle relaxers (of the safe variety, NOT soma/carisoprodol) have been helpful in most, also might try anti-epileptics that have pain reducing properties (gabapentin, and pregabalin).

Anti-depressants with both pain reducing and nerve damping down properties (AMITRYPTALINE) would be my first choice.... oh.. .it also calms down the digestive tract. Obviously, since it hits all the issues, it's the top choice.

But then, there's the psychological possibilities. Addiction/coming off narcotics (especially methadone!) produce a lot of psychological effects: panic attacks and depression. Chemical imbalances are the most treatable forms of depression and any of the SSRI's would be quite helpful; amitryptiline also works, but it is a less effective anti-depressant (but still working on the majority) than newer agents. On the addicts I see, this is less true and psychological interventions are the main thing. Cognitive behavioral therapy is a good start (an intervention in which you identify the thoughts/feelings/assumptions behind your situation and examine them). There are online versions of it. But, it needs to go up another dimension. If I were recommending it for you here's where I'd go with it.
1) don't do it; start with getting any illness ruled out. Then continue with treating the symptoms... basically do all the first section.
2) assuming you aren't mostly fixed up by the first section, you'd know you don't have serious physical or mental issues going on. This is an essential first step.
Then, read up on methadone addiction and consider whether where you are now isn't totally superior to where you were going!
3) In this POSITIVE FRAMEWORK (again, essential) look at the symptoms again in the positive framework. Begin exercise. Feel worse! But, the pain of the exercise within a positive framework, damps down all the other symptoms. It forces you to mentally and physically get used to them and they fade. a lot.

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Brief Answer:
tests are pretty good....

Detailed Answer:
Ok, if iron level good, and stools heme negative (on 3 tests) then not likely to have a problem from ulcers down to hemorrhoids (and polyps unlikely too!). Then.... it's incredibly common to have stomach issues that are minor and often motility related and with an entirely negative on all tests including EGD. Prilosec still helps even when there is nothing like a stomach ulcer.
And... what I often have my patients do, is if there is both a very real issue for a specialist, AND a very real possibility that things will get better and not need one, is to make the appointment but be ready to cancel it ! (and in the meantime, getting evaluated for bleeding and if no bleeding and symptoms gone.. then.... not much is going to come from the gastro visit).

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