False positive ???

This isnt about me. My best friend went to the doctor today and they tested to see if shes pregnant she hasent started her period its been maybe 2 or 3 weeks the DOCTORS test came back NEGATIVE! But, she took one last week on saturday morning and it came back POSITIVE!?? Also it was the cheap $1 test... it came out very faint positive thats how mine came out and now i have a 9month old lol but shes confused why did the doctors come back negative and the one she took Saturday came back positive????

Whats going on.