@Sams99,
Did you have unprotected sex after your last period? Are you using any type of birth control? When during the previous month did you have sex?
IF you generally have a regular 28-day cycle then you would have ovulated around the 17th of the month. You were potentially fertile from 5 days before that to 2 days after (say the 12th-19th of Feb). March 11th is well past the first date of your next cycle and should be strongly positive if you are in fact pregnant.
For a pregnancy test to give a false negative result you'd have to take it within two weeks of getting pregnant, which would have put your ovulation time much later than the 17th. It would be closer to the 25th. Did you have unprotected sex in that time frame?