Since the day I first got my period, I have had my period come every 35 days (even using an app to track it.) But since 3 months ago, my period has been coming every 2 weeks. I am not at all stressed (in fact, my life has recently become a lot better,) I maintain a normal diet, and I am not on any medication. Why is this happening? I am 20 years old and my first period was at 9 years.