update: reading NST strip. contractions on bottom?

Baby's heart rate is on top but what is bottom line? My nurse got pulled away. 
Update: I'm 28 weeks. One nurse said they look light and very inconsistent and one nurse said none at all. I'm being held for observation.
They sent me home and the pain is worse. No UTI, urine is clean. I'm really worried. I never had anything like this with my first pregnancy and I didn't go into labor on my own; I was induced. I already had a lot of the symptoms on the preterm labor sheet they told me to look ok for so I don't know what I'm looking for except to go into full blown labor if that's what's happening and I'm so early!