I took postinor 2 tablets all at the same time after an unprotected sex .a week later i saw very small discharge of blood . what does mean?

Postinor is: levonorgestrel which induces spontaneous abortion otherwise known as miscarriage. I would expect some bleeding afterwards when the uterus tries to dislodge the fertilized ovum and sheds the built up endometrium that is a normal part of pregnancy. You should bleed then, and sometimes it takes up to a week or two. I would make sure to anticipate the spotting for at least a week.