My period was on 18 July and I had unprotected sex before two day of my period and taken I pill within 72 hours and after one week there was light period.. am I pregnant ?

Are you? How do you think we can tell? Did you get your period in August? If not, have you taken a Home pregnancy Test?

If you want to know CAN you be. Then yes its possible. No form of birth control is 100% effective. But if you are late it could be side effects of the I pill rather than pregnancy. You won't know until tested.