Cervical length at 18 weeks vs. 20 weeks

I had my anatomy scan at 18 weeks and length was 3.9. I’ve been having some weird symptoms so my doctor had me to a cervical check at 20 weeks now and it’s 3.4. She wasn’t concerned because it’s still a normal length but I feel concerned about how much it has reduced in 2 weeks.. Does anyone have experience with this?