Gabapentin can cause fecal incontinence.

Lefthanded had this problem. You might message her for
more details.

Yes it is true that acid blocking drugs will affect protein digestion and make more gas, due to bacteria fermenting the undigested protein.

Also a big culprit can be fructose. So avoid all fructose for a couple of weeks and see if it is better.

If this all fails, consider: Are you eating low fat foods with Olestra in them (chips etc)? This will liquify stools. So will Alli the OTC diet pill.

Check any dietetic candy you may be using. The ingredient sorbitol is a laxative. Hershey's is even putting it in REGULAR candies now, so read all your labels to see if you are getting sorbitol. Sugar free gums, mints included.

Also becoming incontinent may be a result of cauda equina syndrome...a stenosis in the lumbar region. This is considered a medical emergency and should be evaluated ASAP.
Loose runny stools suggest something else, however.

Cymbalta can also cause diarrhea. But if you have been on it a while, this is not likely. It mostly does this side effect at the beginning of treatment. (increases serotonin in the GI tract).

Another rare cause could be Carcinoid syndrome. It would be a last resort if other solutions do not work. Carcinoid would be made worse with Cymbalta.