Back pain??
Like what the first poster said, could be a UTI. You can get them quite frequently while your pregnant. I had one and didnt even know, they did cultures and the Dr was totally dumbfounded because I wasnt or didnt feel the pain.
As for relief, heat packs, and have someone rub your back etc...pressure from the shower too will help. Plus dont forget organs move up and get squished as you get further along too. If this is your first pregnancy, things are gonna hurt a little more than consecutive.
I would just call your Dr office and see if they can do a quick test to see if it is a UTI. If you do it is simple fix, antibiotics and pain medication. If they give like Loritab or anything like that it is safe to take. Really. Otherwise they wouldnt prescribe it.