Has this ever happened to you?

Emah
I've had four faint positive pregnancy tests. I went to the doctor today to confirm pregnancy (I would be exactly 5 weeks today), and the doctor's test came back negative. He proceeded to explain that most women get positive tests 10 days after their missed periods (I'm between 8-9 days late). 
Today I'm experiencing some mild cramping and I had a little spotting. I might be miscarrying, but I'm still confused as to why I'm receiving faint positives at home, but a negative at the doctor's office. 
*Note: this pregnancy test was taken 4 hours before my doctor's appointment.