What Causes Delay In Menstruation Other Than Pregnancy?

I am a 19 year old female and I am rather worried because my period is late and has been for the past 2months but each time I do a pregnancy test it comes back negative. I have always had troubles with my period but my own doctor can't seem to find anything wrong with me. I can only have a period when I am on the contraceptive pill which brings it on, but I can't continue using it as it makes me feel sick.