I just went to my L&D last night for back pain, worried about preterm labor, because I had it with my son, and it turned out to be nothing, thank god. If you're worried and it's enough to mess up your day, just get it checked out!
Ring your local L&D....I don't think you should be having it...I know back pain in pregnancy could be normal but I've heard lower back pain can be a sign of pre term labor.....hope you feel better