Cervix shortening

I am currently 26 weeks with mono/di girls. I go every two weeks to check the length of my cervix. It's been good this whole time up until today. Two weeks ago it measured 4.9cm, but today the longest she measured it at was 3cm. They hooked me up to the monitor to see if I was having contractions and I only had a very minor one within 20 minutes. But, of course, I am a little worried to go into preterm labor now since my cervix is shorter. I need some reassurance from some twin mommies that went further than I am now before giving birth with a shortened cervix! The doctor didn't put me on 'strict bed rest,' but I'm afraid to do much of anything since that changed dramatically within a short period.