I have a tubal ligation, i've had reg periods that i could set my watch by, my period is due, but i'm spotting old blood. very abnormal for me! why?

Could be normal: Variation. Other possibilities - hormones are off due to stress, too much soy, gaining or losing weight for instance. Pregnancy is possible after a tubal ligation though not very likely. If a woman has only spotting in place of period or a much lighter period than normal i recommend checking a pregnancy test just in case. If negative it's ok to watch and see if it goes back to normal next mth.