What are the early signs of labor?

I just want others’ opinions or experiences. I am currently 37 weeks pregnant with my 3rd baby. For three days now, I have been having moderate “period type” cramping. It’s not coming and going. It’s constant, and it wraps around to my back (kidney area). It never eases or gets worse; it’s just a dull ache. It’s not taking my breath away; it’s not making me need to stop and take a minute… It’s just there. It makes me want to lay down with a heat pack on my pelvis. So I called my midwife. She had me try Tylenol and a bath. I tried it; it didn’t help. So I went to labor and delivery to get checked. They said I’m not having any contractions. They say my uterus is calm and quiet. Today, I’m still hurting and cramping. I want to lay down and just sleep. It’s driving me nuts. Could it be stretching? Braxton Hicks? Anything showing signs of possibly gearing up? I know I’m not IN labor, but could this be a sign it’s closing in? Did ANYONE experience this before going into labor?