Pregnant with twins

So I'm pregnant with twins boy and girl. I have a 6 year old. Well its been awhile since I have given birth obviously. I'm 26 weeks and for the last week I have had shape pain around my bottom like my belly button down to my pubic bone. A lot of pressure like I'm being torn in half... I have an ultrasound tomorrow and an ob check for the ultrasound but my doctor has checked my cervix or anything lately. Even as I'm sitting I have this tightness around the bottom of my belly. Help me