Cervix already opening at 21 weeks

Today I had a follow up appointment from my last ultrasound when I found out the sex of my baby.  Turns out that my cervix has opened a little more from a week and a half ago.  I'm freaking out because now this is a big risk because if it opens more I can possibly have a miscarriage.  Has anyone else gone through this? What can I do to prevent it from opening more? My doctor wants to do another follow up appointment in 5 days to make sure it doesn't get worse.