What Causes Delay In Menstruation?

My expected period was today.. they are always like clockwork so if im a day or two late i get pretty worried. i have not had my period so far today. im having just about all the signs of pregnancy. such as mood swings, sore breasts and nipples, more veins in my breast area than normal. i took a pregnancy test last night (clear blue) it came up negative. could it be a little to early to tell?