As far as I have gone through this topic online and researched, "low back pain" seems to be the proper medical term corresponding to the issue.
"Lower back pain" is sometimes used in synonymous with that, but in proper medical terms, that doesn't seem to fit in.
Even Wikipedia, WebMD, and Mayoclinic refer to the condition as "low back pain".
Even if I hit Google with the search condition as "lower back" the first response i get is "low back pain" sourced from Appollo Hospitals and others.
Checkout the links here: