Hello and welcome to MedHelp's forums. Thanks for your question. We're glad you are working with your doctor although we aren't sure what they have really done to try to diagnose you. Our suggestion is to see a gastrointestinal doctor at this point. Persistent and chronic diarrhea should be investigated. Stool samples are very helpful but normally three need to be collected to get an accurate picture if the first shows no issues. Things they will look for are obviously parasites as well as bacteria. Something else to consider that can cause diarrhea is different medications. Perhaps with your arm surgery, you've been on medications that are related to this on and off again occurrence. There are conditions like Irritable Bowel Disease and Ulcerative Colitis or just Colitis.
Certainly stress can play a role in diarrhea. But I'm not sure if you have had enough evaluation to determine if that is indeed the case. If this does turn out to be the case, speaking to a counselor may help. If you have anxiety, treating it with therapy and medication may help.
Here's some more info on diarrhea https://www.webmd.com/digestive-disorders/diarrhea-causes#1