How late into pregnancy can it be before a positive finally shows up?

My last period started on January 30th which means my period in a week and a half late (according to my average monthly cycle). Which would put me at 51/2 weeks pregnant. I am getting head ache, im very fatigued, and my are very painful (which has never happened to me before my period starts). But I have taken several home pregnancy tests and also this past Monday I had a blood test at my Doctor, they have all been negative, my doctor wants to retest me this Tuesday. So I am wondering it any of you have had a similar experience and when you finally got a positive result?