late period, bfn, any ideas???

i have a 26 day cycle usually very regular this month i had spotting on day 12 and 13 followed by quite bad cramping on day 20 now i am 8 days late, i done a pregnancy test day period was due but it was negative i also done 1 when i was 6 days late also negative! could test be wrong? has anyone else tested this far after period was due and got negative but was actually pregnant? how long did it take before it showed positive? sorry for all the questions just feeling slightly frustrated i have been to docs and he said wait 2 weeks then test again!! 2 weeks!! easier said than done! i have had itchy sore nipples for about a week dont know if that is relevant or not, if test is right and i am not pregnant where is my period? any help or views would be appriciated