When to call the dr

With having a short cervix i feel like it comes with more aches and pains and pressure. But when should you be concerned and call the dr or be seen in labor and delivery for monitoring? My doctors have not specified.

My last cervical length was yesterday @ 24 weeks and it was 21mm. My doctor has threatened to take me off work if it continues to shorten (I do hair and stand on my feet all day long) I have shortened my days in hopes of it helping. Today I worked about 6 hours and have been having off and on back and side pain since about 4pm not almost 10pm. It feels nothing like labor with my burst but idk if I should be concerned