Extreme back pain and cramping?

mojen
So I'm 39w&3d, last night before I went to bed around 11pm i started cramping mildly , by 3am it was all in my lower back and it hasn't eased up a bit . It's constant period like cramping and when I walk it feels more painful . Is this the start of labor? My doctor said I was 1cm and 100% effaced yesterday. Has anyone's labor started like this ?