Perhaps you brought on Braxton hicks contractions from the lifting. Usually it is not that easy to throw ourselves into true labor (just ask all the women who go past their due date who are willing to do anything to make it happen). But around this time frame, Braxton hicks can definitely start occurring. And that is a lot of weight, perhaps you did also strain your muscles. I would even when not pregnant! I would give your doctor a call just to run it by them and them try a heating pad on your back where it is hurting. Take it easy for a couple of days. Let us know how you are doing!!