If you have a period then take a plan b after sex and get a full period again a couple days or a week later are you fertile again ?

Yes: The bleeding after Plan B in this case is withdrawal bleeding and not a regular normal menstrual cycle. Some women will continue with a normal cycle and ovulate. So, if you have unprotected sex again, you will have to use emergency contraception again. It is est then to stay on a birth control option so you do not have to take the risk of unwanted pregnancy.