16 Days Late

Katelyn
Ever since I've tracked my periods on this wonderful app, it's been able to tell right when my period will come. Off by a day or so but never by much. 
I am not taking any form of birth control, my boyfriend and I do not use protection and haven't been in the 3 years we've been together and we have never had a close call or scare. (My mans pull out game is strong 😉) I have had no change on diet or any exercise.
I am now 16 days late, and it's grown a worry. I took an EPT pregnancy test ( when I was 13 days late)  and it came back negative. My boyfriend suggested to take another in a few days, just to be sure, and I will. 
My question is: what other reasons could there be for my period to be late? I've read stress, and that's what I think it may be, work has me that way. 
Thank-you for taking the time to read and answer this. 
--Katelyn