Stress is a well documented trigger for autoimmune disease, so it's not unsual be be diagnosed following a stressful event. Breast cancer followed by chemotherapy would certainly meet the criteria for a stressful event. For that matter, RMSF could be considered a potential trigger as well.
Many people with autoimmune disease are diagnosed following events like this. It doesn't necessarily have to be an illness. For instance, I was diagnosed with RA after my mentally ill 23 year old burned my house down. Like you, I had a tick born illness as well (Lyme) shortly before diagnosis.
People with RA prior to chemotherapy may see some resolution of symtoms during treatment due to the immunosuppression that occurs with certain chemotherapeutic medications. I was a heme-onc RN for years before moving to critical care years ago, and saw this several times with chemotherapy. Unfortunately, symptoms usually come back with completion of treatment.