When I was about 20-24 wks along my dr told me the same thing, they thought my cervix was shortening, and told me to do NO lifting, stay resting, and off my feet, no sex.. etc. etc. Well, We moved back to my home town shortly after that, and so I saw the dr, I have had most of my life, they check everything out, and said it looked fine. (they've been keeping an eye on the situation) The other dr's were really flipping out about it, and it really scared me!
you said you went from a 14 to a 13.1 ? What measurments is that?
Mine were in centimeters, and everything I've read online about it says that normal cervix length is about 3.5 -4 cm. Mine is about 2.5 cm right now, but I've read that they won't stitch anything (do a Cervical Cerclage) until it's 2 cm, or under.
Hope that helps, and don't worry about it! I'm sure your dr's will keep a good eye on your situation.