How do you know if it's labor?

I am 36 weeks with my third child and have had a very bad backache since 1:30 yesterday, plus some contractions, nothing consistent though. They feel like gas or menstrual cramps. Also very bad indigestion, extreme fatigue and feeling the urge to go to the restroom but not being able to. My doctor just told me if my water breaks go to the hospital. Helpful right?I have an appointment later just wanted to me input while i wait. I am just not sure what to expect because with my first two children i had an epidural before i ever really realized i was in labor. (induction with the first and water broke before contractions with the second) I work an hour away from the doctor or hospital and would like to atleast be aware before it's too late!