Well, hrm...
She could be emotionally overwhelmed...
But frankly asking someone to marry you isn't anywhere near the same as say, let's elope right now, or even starting to plan a wedding. I know some people that were engaged for 2 years before their wedding. They're still happily married 9 years later.
Engagements can be broken off, there isn't that sense of momentum to overcome that one feels in the middle of planning a wedding and you change your mind.
Frankly, if the woman I chose to ask to marry me after 6 years didn't know within 48 hours what her answer would be, I'd make the answer for her, make the decision that the relationship was then at its logical conclusion, and start making plans to return the ring and move on.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."