So confused?

Okay call me crazy but I'm 26 years old had my first. At 13 so obviously I've been having my period For 13 years but I'm having the biggest brain fart in history... my last cycle was started on November 29th and ended December 5th so my question is, is my next cycle December 29th or January 5th like what would make it the 30 days cuz it should be December 29th but I am late so I don't know I'm just having a brain fart I guess thanks ladies