Pregnancy tests are very tricky and often times if they are a low grade test, they will give you a low grade result. The more expensive at home tests can usually tell you about 4 to 6 days before your next cycle if you are pregnant.
If you had a full menstruation, the chances of a pregnancy are zero. During and 7 days after a period, women don't ovulate and without an egg, you can't get pregnant. You don't need a pregnancy test spend the $20.00 on condoms. AIDS kills!