31 weeks and my cervix is thinning...

Amanda • Happily married for 13 years ❤️ Mommy to 8 beautiful kids
I had an apt yesterday and my cervix measured 3.3 cm, it was 4 cm six weeks earlier. My baby was also measuring over 3 pounds at my 28+4 apt.My doctor didn't put me on bedrest or give me specific restrictions, she just told me to 'take it easy' and 'rest whenever you can' Ok, I have 4 other little ones at home so taking it easy and resting really aren't in my vocabulary! Since yesterday I've been making an effort though. Doc also said that baby will probably come a couple weeks early instead of on her January 5 due date. She didn't seem concerned, just said we'd keep and eye on it. But then she said 'let's try to keep her in there for at least four more weeks' I'll only be 35 weeks then! 
My questions: how long does the cervix take to thin out before dilation starts? Since she didn't give me any restrictions, will my normal activity cause my cervix to thin even more? What precautions should I take? Of course I was shocked to hear this so I didn't think to ask these questions while I was there and my next apt isn't for two more weeks. I'm pretty nervous and anxious the more I think about it!