What Causes Delayed Periods With Tender Breasts?

hi... i have been off the contraceptive pill for one month now and the month before that i wasn't taking it properly, i was due my period yesterday yet it never showed...i took a pregnancy test today and it showed up negative but i cant help but not feel like myself...my breasts have been very tender and sore to touch around the nipple area for the past week and have slight sickness feeling (altho nothing over the top) ...could i be pregnant?