Had sex last month, neg. pg test one day after period started. didn't have sex this month if i miss my period this month what could that mean?

Missed period: If a period is missed this month, you want to check a pregnancy test again.

Pregnancy: If you have unprotected sex and miss your period for a week or longer, you should take a pregnancy test as you may be pregnant. Teens commonly have irregular periods - stress, fatigue, recent illness are possible causes. If you had a normal period after sexual activity and have not been sexually active since, pregnancy is very unlikely. Always use birth control to avoid pregnancy.