What Causes Abnormal Reduced Menstruation?

I am doing breastfeeding for my 18 month baby After my son born I don't have my periods for one year after that I use birth control pills and have my periods regularly then I stop the birth control pill then I have periods regularly for three months now dec20th my last period date.but this jan4th I have my periods I have it two days my third day is very light period what is the reason.