19 1/2 weeks
If 8/25/11 is the first day of your last period, you are 19 1/2 weeks pregnant, with a due date of 5/31/12.
Since you mention there is some confusion, did you have an ultrasound which says otherwise (1, 2 or more weeks difference)? That might change your due date and cause some confusion, but we need more specific information. Please speak to your doctor about this.