Duvadilan (Isoxsuprine)

I went to my ob yesterday because i was having mild contractions (i am 26 weeks and 2 days). After the examination (ultrasound and ie), she said i had an infection (i use napkin because when i cough or sneeze i pee a little). I was given vaginal suppository and isoxsuprine and was told the latter was to avoid preterm labor. My question is if its supposed to avoid preterm labor, why am i having contractions? Although the difference between contractions aren't close, it still worries me. Can anyone here help me? I've already sent a msg to my OB and she hasn't answered yet. TIA