Incompetent cervix

Hey guys so I had my first son at 25 weeks due to preterm labor. I am currently 12 weeks and they are keeping an eye on my cervix. My doctor said as long as my cervix is above 3 I’m okay. Went I went almost 2 weeks ago my cervix was already shorter and thinner than she expected it to be. It measured at 3.3 so she said we would check it again when I come back in 4 weeks. I can’t help but worry that when I go back it will be under 3. I can’t start injections until 16 weeks. Anyone else went through this before?