What Causes Delays In Menstruation?

i am nearly 3weeks late on my period, after a week or so of being late i had spotting and stomach cramps and i feel almost a twitching sensation in my lower body at times, i only had the spotting once, i also noticed that the outline of my nipples have become very dark with a very dark blue vein whic has become ver apparent, my breasts are only a small amount sensitive but feel very heavy and slighly swollen at times, i have also have alot of headaches within this time. i took a pregnancy test but it as negative