Am i pregnant should i beleive the dr?

i got avery light period 14 days ago , which lasted 4 days, so i went to the doctor , i had an ultra sound and he told me that he thought this was implantation bleeding and that this isnt ur period bec the uterus walls are thick and that he saw a begining of a pregnancy(a dark patch or somthing) , so i keep on taking hpt's first thing in the morning but untill now its negative , whats wrong , its been 10 days since the last day of spotting . why are the hpt's still negative?????