on cloud nine = very happy. (Informal.)

In a state of blissful happiness...........

It means she's elated and feels that she's on the highest cloud. Clouds are divided into classes and types depending on their shape and height. Cloud 9 is cumulonimbus, which is the highest a cloud can be. That is where this phrase came from.

Heaven is in the sky, isn't it? If I am in the clouds I am getting close to heaven, aren't I? Perhaps cloud number nine is significant of something but I know for sure how I feel about getting this close to heaven. It is ecstasy, a feeling so strong that I would suffer a lot just for a moment on cloud nine. A guy in California bought a lottery ticket for one dollar and won two hundred million dollars. He's on cloud nine. Good Q!

Happy as though you were floating on a cloud! Not sure why it's the 9th cloud though... :)