Hi Jellycat, 

Have you seen your GP about the tenderness? Do you have any other symptoms?  And have any medical professionals undertaken any tests? Have you had any injuries recently or knocks, or any strains e,g, sports, falls, moving heavy items, or bad posture e.g. sitting at a computer without breaks.

I was experiencing discomfort under my right rib for some time then I experienced tingling under my right arm pit, there are lots of organs on the right side so I was thinking could it be my liver or my kidneys and had to press for tests that came back fine, my rib pressure and discomfort was getting worse so I had to press my GP for blood tests they found I was low on iron, B12, and folic acid.  I was getting worse so went to A and E, they suspected back issues, they did a chest Xray which was normal, they should have referred me to have CT scan but didn't.  Then a big mass one day just appeared out of nowhere on my breast, I went to my GP immediately and they referred me to the hospital, they did a mammogram and biopsy and found I had inflammatory breast cancer that had spread to my lymph nodes, I had to keep pressing the hospital to do a CT scan to ensure it hadn't spread any where else or that I didn't have another unrelated issue, they did one after my insistence and found the cancer had not spread as far as they can tell and no other issues.

Therefore, my advice is there are lots of various issues related to rib tenderness/pain usually it can be a pulled muscle but as there organs under the ribs if you haven't already I would seek advice from your GP and ensure they do tests and if they can't give you an answer and you are still having issues insist on a CT or MRI scan if you are experiencing discomfort.,  As said before usually it can be as simple as overstraining or a muscle strain and hopefully it is nothing untoward but I wanted to tell you my story because some times it is something more serious and unless you are assertive if you know something is wrong with your body then usually the health professionals won't do tests to rule them out.