Freaking out!!!!!

Ok ladies so I am 31 weeks and 5 days I went to the doctors yesterday and got some disturbing news I am already half a centemeter dialated baby boy is head down the doctor said he is measuring small so they r going to do an ultrasound on the 14th this is my third pregnancy should I b worried about early labor my first was born at 36 weeks and my 2nd was at 39 please someone ease my mind