HELP. when to take a pregnancy test

I'm having some early signs of pregnancy that are most likely caused by something completely unrelated but I want to take a pregnancy test to be sure. I know you are supposed to take it a week after your missed period but I am on birth control and only have a period every three months so when should I take the pregnancy test? I had sex on the end of my last period so would just taking the test five weeks after that be good?