gastrointestinal issues are pretty common with opioid usage, because of the hard constipation and slow movement of the bowels it is kinda bad for your system, although severe issues don't appear without long and heavy use.
if they surpress your apetite a lot and you eat less that contributes too to the whole feeling, or in other situation when withdrawing there is a thing called narcotic bowel syndrome, I have it almost everytime I withdraw for 2 weeks.
if now I decrease my dosage I also have GI issues again.
I would advise a healthy diet, small meals but eating often and try some laxatives or make yourself go to the toilet more. if you have a prescription from a doctor you can ask him for a script for laxatives and some other stuff for the stomach and bowels.