Important question

Hey everyone! 
So I had my period on oct 20 and my cycle is 31 days so I am now 9 days late at the beginning of November I had a week of brown discharge I took to home pregnancy tests and the came back negative I don't have any pregnancy symptoms I'm not stressed...
But I also just started my first job I usually sleep in and I graduated last year so I went from doing nothing to being on my feet waking up early running back an forth
 (I started this job nov 3)
Could my period be this late because the big change in my schedule or do you think I might be pregnant 
Don't just view and not reply any thought on this might ease my mind