Brown stringy spot before period, on ocp

My last pill of this month's ovral-l is due today, 6th of August and I didn't miss any pills this month or even before. I started ovral-l in December. I've never missed a period and everytime something felt suspicious I took home upt and serum BHCG test too which were all negative. Usually I get my periods 2 to 3 days after my last pill of the 21 days pack but this time, I'm seeing small brown stringy substance in my cervical mucus. It is scanty, thick and stringy and is not even proper spotting. My period is due in 3 days, on 9th August. Is this brown spotting a sign of pregnancy because I did have unprotected sex 2 days before ovulation but there was no ejaculation or insemination. Could it be due to stress because I'm extremely regular with my ocp, to the point where I'm never even late in taking a pill or starting a pack.