My Period Usually Comes Around The 30.day Of My Menstrual

My period usually comes around the 30.day of my menstrual cycle. I've had my period on Dec 15. It lasted 6days, but on dec 31 I've got it again. Lasted six days also. I don't understand why I got my period on dec 31 and what scares me is that I feel like I might get it again so soon. My breasts are sore,I feel as if I might get it again. My periods are usually late, my cycles aren't this short. Can you please help me? Thank you!