My lmp was 1/07/12. Had sex on 15/07/12. Today my usg says I'm 7wks 6days pregnant n my edd is 09/04/12. How is this possible

Because the medical tests are just rough estimates acurate to only plus or minus 2 weeks.

They also count from your last menstrual cycle, which adds 2 weeks. In reality, if you ovulated when the majority of women supposedly do, you're actually 5 weeks 6 days pregnant. If that makes sense. :)