Can a very early blood pregnancy test that's neg. come back later as pos?

last period was jan.30-feb. 3 ovulated the 19th and 20th. I should start today or tomorrow. got a blood test yesterday. I know I was jumping the gun getting test but I really have been feeling different.Should I just accept the neg. or give it a few more weeks.Doctor told me I could retest in two weeks if I haven't started.