Short cervix

i just went to my ob yesterday because i noticed that brown discharge came back. i had that when i was 7 weeks pregnant and treated with duphaston. i'm now in my 18 weeks. My doc checked where it is coming and she said its from the placenta but she also noticed that i have a short cervix which is only measured 2.6 cm. I think the normal is between 3.5 to 4 cm. she said i'm at high risk of pre term delivery and she prescribed again duphaston and also got a jab to prevent premature delivery. i will meet her again next week for u/s and and another progesterone shot. she also ordered bed rest. Anyone here who had/having this experience? Its quite scary but good thing she was able to see immediately.