Home pregnancy tests can register as what is called a "False negative," which means that even though the test says you are not pregnant, it could be wrong. Go to your gynecologist and get a blood test for hcG levels. This is a hormone that will appear in your blood stream if you are pregnant at least ten days, and is much more exact, if you want your proper answer.
Also, if you think you could be pregnant, or want to pregnant, start taking a vitamin called Folic Acid right away, to prevent neurological malformations and disablities in your child.