Dynamic Cervix

Has anyone heard of this or been diagnosed? Just had our big anatomy scan at 21w1d. Baby measured about 9 days ahead and looks great! Only concerning thing was my cervix seems to open and close a bit and changes. They called it dynamic cervix. The shortest it got when we were watching was 2.8. And they say they are concerned at 2.5. But that seems awfully close to me! We go back in a few weeks to look again. Anyone else have something like this? I’m trying not to freak out. 😳