What Causes Delayed Menstrual Periods?

I m 27 years old. I am not having my periods from last one month and approx 20 days. During periods clotting,heavy flow,loose motion and sharp pain in abdomen to lower part of body is common. In APRIL2010 my periods came before 8 days. i have gain 8-9kg. in last year and now i am 58 kg and height is 5'2". In jan 2011 I have heavy flow of white lubricant. Please tell me what would be the reason of delays of my periods