My husband has had severe gas, bloating, and constipation ongoing since even before his diagnosis. The morphine based drugs make it worse, so the more of that he's taking, the worse it gets. You might talk to your doctor about a change in pain med regimen, something longer acting or not morphine based (such as a patch that lasts 2-3 days instead of an oral med).
As far as constipation and gas, my husband does (all are OTC):
Milk of Magnesia (laxative)
Colace (non stimulant stool softener)
fiber supplement and eats high-fiber cereal for b'fast
Sometimes prunes or dried apricots in the cereal to help things along.
He does all of the above daily as needed, usually at the max dosage of the OTC (it's what his GI doctor suggested).
Hope this helps!