Had unprotected sex in april..period 31 days late and counting had negative pregnancy test 3 weeks ago..should i take another one?

Yes: Take another pregnancy test--if it is positive, you have your answer. If it is negative, then you are very unlikely to be pregnant--modern urine pregnancy tests are quite accurate (but not 100%); make sure you are having safe sex by using both good birth control (hormonal method or iud) and infection control (condoms); see your gyn to get more information and to get started.

Sex and period: It is possible you are pregnant. You may have taken the pregnancy test too soon so you can repeat the test now. Still, it is common to skip a period occasionally. I would check another pregnancy test in a couple of weeks if the test soon is negative and you are still worried. You should also track your periods for a while to see how often it occurs.