Scared

Ok so I went to the doctor I was supposed to be 6 weeks and a day today from my last visit and now he saying I'm at four. Which is scary because my first measurements of baby was around 5weeks. He seem pretty puzzled today is my second day of me bleeding. It actually feels like my cycle even with the teeny tiny clots. Cramping no vomiting and leaking a good bit. He said that my sack was stil intact but I need to prepare for the worst. He is confused on how I go from 5 to 4 weeks when I should have made progress and growth to 6 weeks. He said this is an abnormal pregnancy. Can any one help me understand really what's going on.