Preterm labor at 29 weeks?

I'm hoping someone can help....i'm 29 weeks along and over the weekend started having menstrual like cramps that wouldnt let up. Also was having frequent braxton hicks. Went to l&d and found out i was contracting every 3 min and was dilated 1 cm. They gave me 4 shots of terbutaline to slow contractions and took blood and urine samples. Said i wasnt dehydrated and no bladder infection so they didnt know what was causing contractions. Let me go home after 3 hours because cervix wasnt changing. Also said i had microscopic blood in urine but never said why. Went to see my doc monday and he wanted to start me on an antibiotic because he said the main cause of blood in urine is a bladder infection even tho at thehospital they said i didnt have one. Im still feeling crampy and having frequent, but painless contractions. They are always worse in the evening. My doctor didnt even check my cervix when i went in on monday, just said to come back in a week if im still contracting. What could be going on?