What Causes Vaginal Spotting With No Regular Menstrual Period?

hi dr Jain, i took postinor2 4hrs after unprotected sex. my period was late by 8days and then i had dark brown spotting for 4days. i was due for another period yesterday but it has nt yet come. am nt having symtoms of pregnancy except that my bowel movement is very poor and i constipate. could i b pregnant or should i b worried? Sophia(30yrs), Nigeria