false positives?

I just had my son 9 weeks ago. Took a test about a week ago & it came back positive. Went to the doctor, they did a regular pregnancy test & it also can back positive. But my blood work came back negative.. Has this happened to anyone before? Do you know what could cause this? 
Update: No I haven't had a period yet.
I also haven't gotten a chance to talk with my doctor. They called me over the phone & told me my results from the blood work.