I havent been having much back pain, but I have been having a lot of hip pain, I also stand all day long (Hair stylist). My doctor said its from muscles getting stretched out, that and a combination of them getting tired from the extra weight. He suggested a maternity belt, but Im only 21 weeks and Im not really big enough for one yet. :(
mine came on very suddenly too, a few weeks ago I just woke up and it was there. A warm bath will probably help, and maybe some tylenol.