Cervix "twinges"

Hey ladies :) im 15w3d today and since I've reached 15w I've had twinges in my cervix.. Off and on, wouldn't consider it painful but definitely know they are there.. Happens when I'm sitting down only? Is this normal or should I be worried? I have a 4d on Thursday to find out the gender and just really want to know everything's okay before I go.. I've had a missed miscarriage previously at 8w so I guess I tend to worry :)