The real key to this question is timing. This back pain could easily be contractions. If it comes and goes, you can time when it will come, and you can time this for a while, then you are probably having contractions. Pelvic pressure and low back ache are common in late pregnancy. However, because you are planning a c-section it would be best to alert your care provider if you are having regular contractions. It sounds like you might be close to your due date, so if this pain is coming and going at regular intervals it is time to call your provider.
PS. as much as I professionally object to diagnosing CPD (cephalopevlic disproportion) prior to the onset of labor in an otherwise healthy pregnancy, it would be unkind to assume that I know all the reasons behind a mother choosing a c-section. Choice is a beautiful thing, as long as it is fully informed. Best of luck.