My Menstrual Cycle Has Been Reoccurring Every 19-22 Days Going

My menstrual cycle has been reoccurring every 19-22 days going from the first day to first day of next cycle, for the full 6-7 day flow, starting January , 2016 I'm 43 and in good health taking no medication of any kind. What can be causing this to happen? I have always been regular and have never had an issue like this