36 weeks tomorrow.

What are early signs of labor? I ate dinner tonight and had a horrible horrible back ache up my whole back and my stomach felt really funny i didnt want to even move it hurt so bad i didnt it alot. Now i am laying in bed and i keep having low back ache pain it is very dull feeling and its coming and going every few mins. Could this be a early sign of labor ? Or is it normal? I had my son almost 3 years ago so i dont remember much. This past monday the doctor checked me and i was 1cm dilated.