Feeling stupid

I am on my first child, and I'm 39 weeks 4 days and I am so confused as to what's going on with my body. I am unsure I'm going to know if I'm actually starting labor and I'm still working... And I know it's important to rest during early stages of labor so how did you know? I'm currently have a constant ache in my lower back my belly is much harder than usual, my daughter is active, and I'm nauseous...