What Causes Delayed Periods With Spotting?

I am 2 and a half weeks late getting my period. I did a home pregnancy test a week ago when I was 11 days late and the result was negative. I no have light brown spotting for the past 3 days, its not heavy, mostly just when i wipe. I have no other pregnancy symptoms, and also we have been careful when having sex so it seems extremely unlikely that I'm pregnant but I cant help but worry. I am normally very regular with my period and I have not been stressed or had any reason for my period to be late so I cant help but worry