Have anyone been pregnant but couldnt get a positive test til later?

I had my last period in june twice which has never happened! one at the beg of the month one at the end! the period at the end only lasted three days very light! all of july and aug i have not had a period! around the time i was suppose to come on i cramped for about five days but never came on! my breast are heavier , ive had headaches, tired, and whenever i eat my stomach never feels full! i took a test but it said negative! im getting fustrated! has anyone ever been pregnant but couldnt get a positive test til later?