Late period

Ok so I am new to this but my period was late three days to be exact. I took a pregnancy test yesterday it came out negative, today my menstrual decides to show and I'm wondering what could have caused a late period or delayed rather. With my first pregnancy my menstrual was delayed but then it came on took pregnancy test and the home and doctor came out negative but then I also found out I was three mod pregnant later on so yea I'm confused