With my first pregnancy, I was experiencing extreme lower back pain, but off and on (I felt it almost every day, but some days I could barely walk, and others it was just "there", but was very bearable) and like you, it started very early in pregnancy. My ob thought it was sciatica nerve pain as well, and she said a massage therapist may help, but that it would probably still be off and on through the pregnancy. I never did see the massage therapist, but the pain did subside quite a bit somewhere in the second trimester. And it came back more often again in the third, but I think it was probably not until month 8 or 9.
Sorry I don't have any advice for you, I just wanted to let you know that with me, it did seem to get quite a bit better after the first trimester, until almost the end!