I had this too. As above its the uterus growing and putting pressure on your back. Once your uterus moves up above your pelvic bone it will be able to move forward more takeing the pressure off your back. Usually around 12 weeks.
I had similar pain and my Dr told me it was because the uterus was growing and it can be very sore even to the touch!
It might be ligament pain from the relaxin pregnancy hormone.