Hey

I have got the same problem except back pain is in lower spine (lumbar s1 and another one cant remeber.) it is worse in evening and night then during the day and sometimes almost unbearable> reason lower back pain usually in one side is caused by progestin (hormones) they soften your bones to make them more flexible during birth. I find if the pain is really bad grab two tylenol. I have other symptoms with my back pain and someday I really think im going to loose my baby I just cant wait till im 28 weeks and reach safe zone so another 3 wks to go

Chiropracter!! I've had so much relief from visiting mine. The hormones alone can put your lower spine into a twist. I've recommend it highly, hopefully your health plan will cover the visits :) mine doesn't.
I pay $35.-$40. For a couple of adjustments. My doc is very tuned in, and naturopathic, she will even come to the bith to hhelp me feel more comfy. I can get adjustmests almost all the way up to the actual birth.
Check it out, also massage from a licsensed pregnancy aware masseuse can help too. Skip the drugs, and get help the natural way, your baby will be happier for it.