What Causes Delay In Period, Sore Breasts And Nausea?

hi im a 26/f---LMP 12/8/2010 i have a regular 28 day cycle---i was due to start yesterday---period never showed... i been cramping off and on for about a week now. cramps have subsides but still no period...I also have sore breasts with nipple sensativity.. certain smells make me want to vomit. nausea pretty much of and on all day. I took a hpt 5 days before expected period and was negative. Could i still be pregant?