Ultrasound has been a great tool in ending confusion in situations like yours. You are probably only five weeks along. This is probably because although you remember your last period, you have no idea when you actually ovulated. And if there was a delay in ovulation, this would explain how the size on ultrasound is different from the arithmetic based on your LMP. Gestational age based on an LMP is a traditional method, but is only accurate when women have rhythmic cycles.
Usually, if the gestational age by ultrasound is within one week of the mother's dates, I don't change the due date and go with mom on her arithmetic. But six weeks off begs for a new due date.
(One added note: it's possible that you really are 11 weeks, but the baby stopped developing at five weeks—indicating a miscarriage—a "missed" miscarriage). But a follow-up ultrasound will demonstrate a fetal pole within a week or two, so don't panic. Also, a vaginal probe ultrasound is much more accurate at seeing things earlier than an abdominal scan.)