What Causes Light Menstrual Bleeding?

Hi, i m 24 years old F, got married 9 months back and we planned for a birth just 3 months back say Jan 1 2015, Since we started i m getting decrease in Blood Flow during my past 3 periods.. Anyhow i checked Hemoglobin level, its 12 ..and also i have taken iron tablets for last 3 months.. pls advice what cuases the decrease in flow for the last 3 months..Thanks