Cervical shortening

I have to go every 2 weeks for cervical measurement. I was told that in 2 weeks time I went from 4 cm to 3.5 cm. That I need to drastically reduce time on my feet if I don't want bedrest. I work in a call center and sit all day. How is it changing so fast I'm not on my feet much. But boy every time I stand up my feel so much pelvic pressure. BTW I'm only 21 weeks & 4 days