3 cm dilated and contractions

My sister went to the doctor today and she is 36 weeks and 3cm dilated. Well she started having contractions and went to the hospital. The contractions are 2-3 minutes apart. They are checking her again in an hour. If she's more dilated  then they are admitting her. If she's not then they are sending her home. Has this happened to anyone before? What happened?  Btw her doctor WILL NOT INDUCE HER.