What Causes Delay Periods While On Birth Control?

Hi, I am a healthy 38 year old female who is on the contraceptive pill but I have had no period for 6 months. I am not pregnant. I have two children. I have been for blood test (still awaiting results) but if these come back clear, I don't know where to go next. Am not bothered about not having periods as long as there is no serious problem as to why they have stopped.