Help!

I'm about 3 weeks late and I am never late. I took a home pregnancy test and it came back positive, but when I went to the doctor my blood work came back negative. Could it be too early to tell or could I have possibly had a miscarriage? I started bleeding today and having cramps but I'm not sure if it is my period or a miscarriage. I go to the doctor again this week to get my blood work redone. What do you think it is?