With my first pregnancy and again with my second I have experienced some pretty bad lower back pain starting around 15 weeks. I haven't gained any weight yet so I know it doesn't have anything to do with that. Could my babies sit on a nerve or something? It hurts constantly and nothing helps. It is excruciating at times when trying to sit up or turn over while lying down.
I had early back pain too. Its just due to your uterus growing, but isn't high enough over the pelvic bone to pop outward. As it grows higher it will start moving outward and take the pressure of your back.