Because joint pain has been experienced by patients on Imbru/Ibru and others have not had this side effect, it is wise to explore all possible causes.
I just found this article on a new virus to the US. It has a cool name "Chikungunya" and you thought new drug names were hard to pronounce This virus presents with flu-like symptoms, fever and pronounced joint pain that can persist after initial symptoms subside. It is in the process of establishing itself primarily through 2 mosquito species in the Southern and some Mid Western portions of the US.
Check on this virus if you are in countries other than the US and in warmer areas of the world. Looks like Canada and the UK may get a pass on this one for now, until we can get more CO2 into the atmosphere.