Question about cancer coming back

Hi all, my mama, 75, is a breast cancer survivor.  She has been cancer free for close to 15 years now. She was so fortunate that her breast cancer was caught before it had spread to the lymph nodes. I forget the staging levels but I remember hers being a stage 1 or not even a stage 1 (if that is possible) She had it removed and had a double masectomy, chemo & radiation and then tamoxifan for 5 yrs and Femara for several years.  Anyhow, she also has arthritis and degenerative disc damage and pain in her back and she has had this and been dealing with pain for years and years.

Here is where my question comes in.  I would say for about 4 or 5 months now she has had chronic lower back pain to where I really notice a difference in how active she is which is not much..just cooking a little bit and her back is throbbing and she goes and sits down in her recliner.  She rotates tylenol and lortab daily for pain and ice packs.  Just doing very little, causes her pain..she said it feels like deep bone pain..like her bones are just grinding together.  I have alot of back & neck damage myself so I know what she is talking about when she say that deep pain..sure most of you do too.

My question is, how can I ease my mind?  Am I overreacting thinking this pain could mean some type of cancer coming back?  She hasn't seen her cancer doctor in a couple of years now and I have been wanting her to go.  She got frustrated going towards the end because her doctor would just talk to her and maybe examine her but not run any tests so she figured, why go? (money has been super tight, that was her main reason in not going) If she did go and told her cancer doctor about this unusual, consistent pain, how do they see if it is cancer?  Do they run blood tests?  I really want her to go just to rule it out. She has labwork every 3 months with her primary and each time he does a lipid panel, and A1C for diabetes and that comprehensive metabolic panel and all of these have been coming back good but none of those tests show if cancer is coming back do they?

Thank you in advance for any info.  I am 49 and just love my dear mama so.  She is such kind and giving person, I just want the very best for her and I hate to see her in pain.

PS> I had my baseline mammogram in '06 and it was all good so far.  My mama's 2 sisters both had breast cancer and they had both been cancer free for even longer than mama but just 6 months ago, her oldest sister had cancer come back in the liver. She is going through chemo/radiation and the latest news was that the tumor was shrinking. After my mama's sisters, there are 4 of her aunts that had breast cancer, and my great grandmother had it, and a few of my 2nd cousins have it..it runs in our family bad. 

Thanks again, Kristi