Do you mean you "think" you are pregnant?

Because if the doctor tested you as negative then I guess you aren't.

It is impossible to say when you might have conceived as you do not state how long your typical cycle is.

If we were to assume you had an average 28 day cycle
and started bleeding on 4 dec then ovulation would typically occur around the 18 dec.

Conception would be within 36 hours of ovulation

implantation can be anywhere from 6-12 days after conception.