Shortened cervix @ 30w5d

As

So I've been cramping off and on the last couple of days but tonight the cramps became more intense and I was experiencing a lot of pressure in my back. We rushed to the ER (which is an hour away & there's a blizzard coming) to get checked out and make sure nothing serious was happening. Labs came back fine, my water hadn't broke, I wasn't dilated but the doctor saw that my cervix was shortened. So he decided to keep me overnight, put an IV in, and recheck me in the morning and also get an ultrasound, I'm just wondering what exactly happens when your cervix is shortened. I didn't catch the measurements he gave me but he said it's pretty much half the size of a regular sized cervix