There is usually about 10 days between fertilization and implantation. The embryo does not produce pregnancy hormone until implantation. So, there could be a 10 day gap between the time a conception occurs and the time we can detect it. That is probably why your doctor wants you to wait. If you don't want to be pregnant, be sure and use something during this wait time! If you were my patient and had both a neg test and no MP on the next set of tests--I would prescribe progesterone to start your period--I like to use Prometrium because it won't harm a baby in case of an "oops".
I think I'm going to wait to take my 3rd pregnancy test, in the light of your comment here, but I am displaying multiple symptoms of early pregnancy - missed period, nausea, frequent urination (with more secretion), extreme weepiness, backache, headaches, weight gain on tummy and breasts (which are sore - very much the way they get before period and I think the areola are darker).