Shorten cervix at 18 weeks

I'm 18 weeks pregnant and my cervix are starting to shorten. I am considered high-risk because with my last pregnancy at 16 weeks my cervix started to shorten and I lost my baby at 17 weeks due to my water breaking. I've been very nervous and very precautious with this pregnancy. I see my high-risk doctor once a week and today they told me that my cervix is starting to shorten again I am so nervous.