What Can Cause Delay In Starting Of Menstrual Cycle?

Hi! I'm 13 and I still haven't gotten my period. I have a lot of dischanrge in my undies everyday! It's sometimes even yellow, is that normal? And I have vaginal hair and my breasts are growing yet no period! I am a competitive swimmer and To be honest I don't have a lot of body fat could that be why?! Help!!!!