My period is now 13 days late from its expected time. I'm 17 so i know it's not regular, but my cycle usually lasts about 37 days and now I'm on the 51st day of this cycle. The last time I had sex was more than 3 months ago and I've had my period 2 times after that, so I'm sure I'm not pregnant. I have been under extreme stress and have been worrying a lot. I also stopped my regular exercise this past month.