HELP!! Urgent

I had unprotected sex about 8 days ago.. I took plan b 2 days later.. I went to the doctors yesterday to get STD tested and found the pregnancy test came back positive.. I panicked and asked for the doctor to do it again. This time it came back negative. The doctor said she still thinks I'm pregnant.. It maybe me in denial. Not sure what to do? Or is their any chances I could actually be pregnant. I've heard theirs never false positives?