What Causes Irregular Periods?

I'm 18, 5ft3, weigh 7 1/2 stone, have a healthy medical history. But my last period was 6 weeks ago, a spotted for a couple of days 3 weeks later then was late for my period by 2 weeks. When I did get my period it was lighter then normal, and looks darker and stickier then normal. Is there anything wrong with me? Thanks