3weeks ago i had unprotected sex few minutes later i took a planb pill but ever since then i been feeling nausas, very tired, and i been vomiting.why?is it

Common but 3 weeks?: Plan b is taken within 72 hours of having unprotected sex, and then again 12 hours later. Plan b is just a high dose of birth control pills. Planb commonly causes nausea and vomiting as a side effect. On the other hand getting pregnant also causes nausea. It is unlikely if you took the Plan B correctly that you would be pregnant. Nausea 3 wks later is concerning. See your doctor.