What Causes Delayed Period While HPT Is Negative?

Dear Doctor, My wife n i trying for a baby for past one year we got married on 2011 Dec. Whereas my semen analysis was too low so i went through medication for atleast 6 month n nw its normal. My wife went through the HSG test n her tubes are normal as well. Nw her periods has been delayed for 12 dayz. We tried giving pregnancy test in Lab but came up negative. But she feels lik she doesnt have any symptoms of Pregnancy. But out of curiousity since its gettin delayed for this long n this havent happened earlier by any chance she might be pregnant ?. Please advise. Thank You.