how accurate are hpts?

this is useful for us ttc women:
http://www.ynhh.org/healthlink/womens/womens_4_04.html

i'm late by the way, but i don't think i'm pregnant. i just feel an overall sense of shittiness, like there's this invisible cramp lurking in every nook and cranny of my body. i don't even know whether it's imaginary pain. and it's making me crazy. someone give me a pump. i think my plumbing's stuck.