Spotting, why?

Hey, I'm on a pill for 8 months now and sometimes it still happens for me to spot some old blood/brownish discharge like a week before my expected breakthrough bleeding. This happened to me this month after a few months of no problems.

Can this be caused by heavy stress (moving out, new job) and lifestyle changes (working 12hr shifts, exercising, diet) even though I'm on a pill and.. it should not matter that much anyway? Please educate me.

+ no, I did not miss any pill.