My lmp was 2/26/12. based on that, my due date is 12/2/12. us results at 18 weeks changed due date to 12/1/12. is there any way i could have become pregnant around 2/17/12?

Unlikely: A LMP of 2/26/12 gives a due date of 12/2. Given the error that can occur with any ultrasound, the fact that one due date was 12/2 and one was 12/1 doesn't mean much. If you started bleeding on 2/26 and the period was normal and you've had ultrasounds that have given you the due date above, it would be highly unlikely that you became pregnant around 2/17.