Spotting after one week of periods.

I have unprotected sex on 6th & 7th day of my periods this month and taken i pill on 7th day after doing it once for both the days. I have less to no(only spotting) bleeding problem during my period from last 2 years. I have started taking homeopathic medicines for this problem from 8th day of this cycle. Now on 14th day i saw dark brown spots in my underpants and today on 15th day too same happened, plus i have cramp and pain like period from last 3-4 days. Lastly i have taken ipill once before 2yrs back that time no unusual things happened earlier. Now please tell me possible reason for this, i have read on internet that this happens when pregnancy has been established in uterus line or normal ovulation process or just because of stress.