Lower back pain. A sign of labor?

I am 35 weeks pregnant and yesterday my doctor told me i am dilated to a 1 or 2 and my cervix is already softening. She said i very well could have him early. I woke up today with some pretty bad back pain that radiates into my upper thighs. Obviously her telling me i may go into labor early is freaking me out, but I'm not imagining this pain. Could it be a sign of labor?