Cevival incompetence

This is my second pregnancy currently 13 weeks. I lost my first baby due to cervical insufficiency at 22 weeks 5 days. I also had rubella rash two weeks after my miscarriage so the cause if loss is still unknown. Now my doctor recommended me cerclage at 12 weeks which i had 3 days ago. I had my pelvic ultrasound done a day before cerclage and my cervical length measured 3.4 cm. which radiologist thought is nice. But when i went to OT for cerclage doctor told me after procedure that it was very hard for her to put cerclage since my cervical length was not more than 0.5cm. I am literally shocked that how can my length shorten in just 12 hours. And i have no pain bo symptom. No bleeding. No fluid coming out of my vagina. I layed on the hospital bed day before all day and i did not do any heavy activity. Im soooo confused right now how can that happen. With my last muscarriage when my length was shortening i did have contractions. But how come i didnt feel them this time? Is there any one who has this kind of experience? Im so confused and worried. Please share your thoughts about this.