Nothing on ultrasound?

hi my last period was on october 15,I went today for an ultrasound after I had brown discharge(on my underwear and when I wipe).The doctor said that she can see that the uterus has a thick lining which is a good sign but she couldnt see any sac.she told me that fertilization may have occured late and that's why she cant see anything...she said maybe im at week 5 and not 6.I'm very scared (especialy from ectopic pregnancy but i dont really have any pains or cramps)and I wish that you share with me any experience or info u know about this