Cervical length and dilation at 20 weeks.....

I went to L&D yesterday because I started having some pressure Friday.  It was really bothering me yesterday so I finally decided to go.  They checked me and said I was 1 cm dialted and my cervical length is 3.1.  I was given 2 shots of turbutaline and it helped the pressure go away.  The monitor couldn't pick up any contractions but they said I could be too small for it to and that the pressure is probably my contractions.  I had PTL with DS at 29 weeks and was 4 cm dilated and 100% effaced within about 3 hours.  I was on hospital bedrest for 5 weeks then had him 2 weeks later at 36 weeks. The on call dr wanted me to stay over night for observation but he let me go home since I had an appt with my OB this morning and I also go in for my a/s on wed.  I seen my OB this morning and she said it is ok for me to be dialated since this is my 2nd pregnancy but it worries me since I'm only 20 weeks.  She prescribed me turbutaline 2 weeks ago to take as needed and today she told me to take it every 4-6 hours.  I was reading online and it says a cervical length under 2.5 is something to worry about. Does this sound normal?  I'm not sure if I should be ok with it or try to see the peri I seen when I was in the hospital with my DS.  The only reason I didn't call him already is because he is a 2 1/2 hour drive from my home.