What Causes Brown Spotting With No Regular Menstrual Period?

Hi, I am a 23 year old female. I have a question regarding my menstrual cycle. I had brown spotting for two days. It was like a thick paste like consistency. On the first day there was a tiny bit of blood in the brown spot and on the second day it was just light brown spotting. After that, nothing! no blood. I have had the brown spotting happen to me before prior to my period but it always leads up to my period. I have never had it for two days and then stop completely . I am wondering what this could be?