My lmp was unknown; from an 8 week ultrasound it was estimated to be 10/15/12. if i ovulated later would the lmp be different on the u/s?

Ultrasound dating: Your LMP can only be estimated by ultrasound. It is based on measurements of fetus/ gestational sac. First trimester ultrasound is accurate within 5-7 days. 2nd trimester ultrasound has accuracy of +/_ 2 weeks. Lmp dating also assumes a 28 day cycle. There is no test that can date conception more accurately than a 1 week window.