Hi:
My story is similar to yours. I had a completely normal first pregnancy with normal cervical lengths reading my medical records. CS at term secondary to non reassuring heart beat. Second pregnancy, I went in at 18 weeks for increased discharge and I was 6cm dilated. Rescue cerclage failed, i ruptured and lost my daughter. My OB ans mfm said I had and still have no risk factors. Currently pregnant again with a cerclage on bed rest.
Sometimes in medicine things can't be explained and it does bother us a lot. Do take it one day atba time, bed rest and progesterone. There is nothing else you can do now. Praying you make it as far as possible. Positive thoughts and vibes.