I think it means pain in the armpit that is unusual/doesnt follow the usual pattern or distribution, or does not react to drugs/painkillers as it should
Well, she has been to the hospital for probs with high BP and heart which were fully explained in this report, but this phrase was added almost as an afterthought. She has to go back for a check up, but wanted to know what this meant. The consultant isn't very chatty and when asked questions tries to dismiss them as though my friend is wasting his time, which isn't very helpful.