Not sure that they are BH - as they are contractions of your uterus, but it could be the ligaments stretching with increasing weight etc. If i walk too far on any given day i get bad back pain on just one side of my back - very low down. Try some sretching or warm baths if it gets too bad.
Oh also back labour usually refers to if bub is posterior (like my cheeky monkey) but if your ob said bub is head down on the let you should be ok.
Good luck and if your still unsure - i would call your hospital just to check!