What Causes Light And Irregular Periods?

Hello Doctor ! I'm a female and my age is 21 years. My query is about my menstruation. The first thing is that I don't get my periods on time...like it is always 10-15 days delayed for eg last time I had my periods on the 22nd of April, 2011 and this time I had my periods started yesterday i.e, 6 May, 2011so as you see it is delayed 15 days is it normal or is there any negative effect ?? Secondly from the past 6 months I'm noticing a change in my periods like the first day of periods I don't get much flow its only a few drops which is coming out the entire day and then the second day the flow starts it never happened to me before but from the past 6 months this is the same situation I'm facing every month.