Sita was thinking if she has experienced happiness at all in her life. While she was thinking like that she started experience good omens
1. Her black, bow like left eyebrows started quivering.
2. Her left , round, beefy shoulders , which was embraced by Rama, started quivering.
3. Her left thighs, equivalent to that of an elephant trunk, started quivering.
4. Her clothing slided down a little.
All of this , appeared as if they informed her that her sadness is going to come to an end and Rama will come and save her soon. Seeing All these , Sita became brightened and appeared like a moon coming out of Rahus mouth. Her face was glowing with confidence and radiance and it appeared like a moon in a bright fortnight.
P.S: Rahu is a Snakes head in Indian Mythology. For more info http://en.wikipedia.org/wiki/Rahu