What are the odds I'm pregnant?

Last monday, I went to the doctor for my annual checkup, my period isn't "due" until tomorrow. Usually, when I'm about to start my period, I have moderate cramps the day before and of. Today, I've barely felt anything. Eariler, when I went to use the restroom, and I wiped, I noticed a little spotting, and it was a dark color. When I went to the doctor, could it have been too early to for the hcg to pick up in the urine, because I know they say your not supposed to test until after you've missed your period. Could this be implantation bleeding? What does the dark color mean?