If my last period was may 25th 2012, an
D I'm ten and a half weeks pregnant, when would I have conceived, normal 28 day cycle??

On the day you had sex, during that 28 days after your last period.

Ovulation days vary by person, so there is no way to tell for sure.

You doctor can give you a estimated day, but it will be normally give or take at least a week, and sometimes could be off 2 weeks