What Causes A Delayed Menstrual Period?

Hi , i have just turned 13 and i statrted my period on the 17 th of december ...i came on the right date in january ( 17) and now i was due on on the 17 of march but i now am 20 days late :L ... and i dont want to talk to my mum about it cause were not really close ... is there somert wrong with me ? :S