The egg gets fertilized but it never implants. Since the egg is fertilized your body produces hcg hormone which will show positive on a pregnancy test. The lines never get darker and ultimately get lighter and you end up getting your period. I had one in August. Tested 3 days before missed period. Had quite a few positives, even a positive digital. Tested day of period to see my lines got darker and it was flat out negative. Got my period the next day.