Symptoms of preterm labour or...?

I'm 29 weeks with bi/bi twin boys. I'm scared of having preterm labour and not understanding the symptoms. It's been two days i feel my belly very heavy, i have discharge much more than before, i have contractions that can last 1 hour every 10 minutes then they stop for a couple of hours. Now i have lower back pain and some mild cramping. Is this all normal or these are symptoms that i have to see the doctor?