Simpson admits they had talked about marriage in the future, but she never thought her partner would propose after such a short romance.
She tells Access Hollywood, "I was so shocked in that moment that I had to sit down on his knee and catch my cool. I did not expect it at all... I just didn't expect that moment to happen so soon. We had talked about getting married, but I didn't know that he'd already gone to my dad three weeks before that and to my mom. Everybody kept it a big secret in the Simpson family, which is not a normal thing."
And Simpson has been so caught up in the engagement, she has yet to consider any plans for her big day.
She adds, "We just really haven't talked much about it. We've really just been happy being engaged, so I don't even know what kind of wedding I'll have. We've both been married before, so you never know what can happen."
The 30-year-old star was previously married to Nick Lachey.