Girls, I think I'm going into labor. Since 4 am, I've had what feels like…

Girls, I think I'm going into labor. Since 4 am, I've had what feels like cramps in my lower back and my belly is pulling like during my period. It's been two hours now and the pain has gotten worse. It feels like there are spasms in my back. An hour ago, the mucus plug came out brown-ish. How soon should I call an ambulance? Maybe already? Or wait another hour? I don't want to panic and go there right away!