I had the same thing. I didnt believe it was ligaments either, though for me it was. If your concerned then you should see a doctor just to rule out anything else.
Stretches and Tylonol helped me. Tylonol PM is approved to be taken during pregnancy.
Good Luck.
I don't want to scare you but I miscarried at 5 weeks and had quite severe back pain. With this pregnancy I got some back pain that almost radiated down my thighs too. For that I knew it was just everything stretching and growing and it's ok to take regular strength tylenol. Don't take advil. As Crystal said...just relax and maybe put a warm compress on your back too.
Stay off your feet as much as possible. I had bad back pain when i was 5-7 weeks pregnant and ended up having a miscarriage. Talk to your doctor and maybe he can tell you if the baby is going to be ok.