Suggest Remedy For Pain On The Upper Rib Cage

My mother has had a pain in her left side, fairly high up sort of rib cage below her armpit and back a bit! It has been a dull ache for ages, her Dr said it was a pulled muscle. But it hasn t gone away. About 3 weeks ago it became very painful for her, so bad she was unable to concentrate and didn t know what to do with herself. I took her back to her GP who sent her for a second X-ray. He also gave her some co co dismal pain killers to take, which do work. But as soon as she stops taking them she is in huge pain again. She is 88 and I am worried this is something serious? We have had to wait for 2 weeks for the X-ray results which we should get tomorrow but I know she can t keep taking co Codamol. What could this be and what can be done to ease her pain please.