I honestly don't know. I had heard that it usually comes much later in your pregnancy. I am in week 33 and still haven't gotten mine. I think it has something to do with the hormones, but I'm not sure. I have a book, though, and I will try to read up on it sometime tomorrow.
You might just be showing the line very early. Please don't stress over it. If you aren't bleeding or cramping terribly then you must be doing very well.
Congrats and keep us posted if you find out anything about it from your doctor.