Implantation Bleeding??

Heather
Most bizarre thing ever. I'm definitely pregnant but the doctors can't tell how far along I am...based on when I had my period last I should be 5 weeks but they suspect I might be very early. Only 2-3 weeks along. Is it possible to have implantation bleeding/discharge for a whole week? What are the signs of a miscarriage? Any advice or input would be so great!