What Causes Delay In Period While Trying To Conceive?

Hello dr. I m 36 yrs old I losty baby in sep 14 by c sec .I have been trying for nxt one for last three months but not succeed. From last month I m taking pregnancare conception. this month I haven t period 6 day past my due date was 17 feb today I take pregnancy test but it was negative I m worry what should I do pls pls suggest me . Thank u