Contractions?

This is my first pregnancy and im now 34 weeks. I started feeling some pain in my back last week and went to the hospital, they told me i was having real contractions and i was having them very often. Then gave me an iv to try and stop the contractions which didnt work so they gave me a shot in the back of my arm which was a medicine to try and stop them. Which it worked. But the thing is once i got to the hospital and laid down i almost left because i didnt feel the pain anymore. But the nurse said i was having really bad contractions.. How did i not feel them? How will i know when i go into labor if i couldnt feel most of the contractions? My due date was July 3rd but they moved it to June 20th because i recently was diagnosed with GD and they are going to induce me but my doctor told me to count the contractions until then but i cant count something i cant feel! >.< help?