Labor

i am 36 weeks pregnant and this is my first pregnancy and I keep having very bad back pains in my lower back. The pains are so bad that it wakes me up at night time and it is so hard for me to go back to sleep. I went to the hospital the other night and they said I was so dehydrated that it was making me have contractions. So they told me to drink more water so I have been drinking a lot of water and peeing a lot also. But these back pains are still here. I am not sure what these back pains are. I don't know if it is a sign of labor or what. What could the back pains be that I am having in my lower back?