What Causes Light Brown Discharge Instead Of Proper Period?

I was due my period Monday which would be a 27 day cycle. I am like clockwork every month however this month on day27 I just got slight brown discharge (my period is usually very heavy). Now I am on day 31 still no period and still have brown discharge. I have done 2 first response tests which came upvery faint and a clearblue digital test which came up not pregnant.