Hi Anna I presume it is a baby you are askin about? It could possibly be a kidney infection but I'd keep a close eye and ring gp if it persists as young children don't normally suffer with back pain,it might be a symptom of something else, hope you get it sorted!!!
Do you mean that someone has had lower back pain for 18 months?
Depending on the age of the person, it is highly likely to be degeneration of the bones. Osteoarthritis is wear and tear on the bones as we age, but can happen for other reasons. This is confirmed by xrays.
Rheumatoid arthritis is an autoimmune disorder as is identified by a blood test.
Apply heat to the back pain.
Make an appointment to see the doctor to confirm the diagnosis and the appropriate treatment.