I had early breast cancer and a mastectomy in March this year no lymph node involvement. It was a recurrence of early bc I had 8 years ago. I have had pain in my back ribs for some time that is not constant but I feel it when I move a certain way. Like you I was very worried about it. I got my physio to check it and he believes it is stiffness where a rib joins my spine. He massaged it and gave me some rotational exercises and it has improved a lot. His view is after a certain age we all have deterioration that is not necessarily arthritis. I am 59 so definitely old enough to have deteriorated🤣
This doesn’t really answer your question but I understand your concern and just suggesting a physio check could see if it is a physical issue. If you are still worried your drs should understand and support you getting scans. I did insist on a pet scan when I found out the bc had come back this year as I was worried about metastasis.
all the best