It could be Unstable Assassin by JoeRhys. Rated M. It's complete with 37 chapters.
Summary: They were at war and she fought for the highest bidder, demon or human. She was no saint. She was no savior. She was no hero. She was the Asa Shin Fujo. The assassin priestess. Her current benefactor wanted a certain Lord of the West dead. Unfortunately for them, they were unaware that their current killer for hire had a secret. A secret that made her lethal and a bit unstable.
I read this at FF.net so I'm not sure if this fic is here on dokuga but here's the link:
www.fanfiction.net/s/11330446/1/Unstable-Assassin
I hope this is the fanfiction your looking for because there could be other fanfics about Kagome disguising herself as a servant for Sesshoumaru.
EDIT BY WICCAN: This story would be rated MA on our site, NO YIMs!
