Is my Yorkie fixing to start whelping?

Question
I bred my yorkie on her 11,12,13th day of heat cycle.  I took her in last Friday for an x-ray thinking we would see the pups.  The vet stated he could see what he thought to be one puppy but he felt it was too soon to see the full bone structure of the puppy.  He also thinks I was ahead by about 6 days on her due day and that you should go by the day of ovulation not by the day of breeding.  Well my yorkie had been acting pregnant, milk in her nipples, lathargic, digging..all that has happened for about 3-4 weeks now.  Just in the past couple of days, I just didn't think she was pregnant because she hadn't been having the "swelly belly" like everyone says.  Well I had her on my lap earlier and I noticed what looked like a long hair stuck to her bottom.  The closer I looked at it, it was coming from her vagina.  It was like a really thick hair and seemed to leave her vulva kind of crusty....she has been licking a lot the past day or two.  My question is, Is this the mucus plug?  What exactly does it look like?  How long do I have from now to start seeing serious signs of whelping?

Answer
Hi Holly. The thing is, unless you do pregesterone testing, you don't know exactly when the dog ovulates, that's why we go by the days breed as due dates. When you say crusty, what do you mean exactly? The mucus plug is a clearish, stringie glob. You'll definitelly know it when you see it. Whelping usually starts within 24-48 hours after the dog looses the plug. It may be hard to see the plug as some girls are really clean.