Codeine: Stomach Discomfort After Use?

[PieceByPiece]

I've been using Codeine for a couple of months now. Sometimes a few hours after /the next morning, I get stomach pain, like I've pulled a muscle? What is this? A side effect? Or something else?

 

[cuivre]

How much acetaminophen/paracetamol are you taking at one time? Is this pain in your upper right quadrant, or elsewhere?

 

[PieceByPiece]

Thanks for replying.
No, I don't take anything with the codeine (pure) apart from either promethazine or diphendryamine.
Yes, it's in the upper right.