sounds like the spotting was right before your irregular menstrual period. since you are not sexually active, this rules out lots of things such as sexually transmitted disease and pregnancy. if you have been exercising more, or dieting, this can affect your hormones and cause your menstrual cycles to be more irregular. and spotting some right before your period is normal as well.
are you worried about this spotting being a sign of cancer?
at your age, spotting like this would not be a sign of cancer.