I think Rumiko Takahashi(creator)put the jewel in Kagomes side cause if it was in her chest Mistress Centipeed would have probably killed her. Mistress Centipeed slashes the jewel out of Kagomes body so if she died there wouldn't be an Inuyasha series.

Kagome was born with it inside of her because she is the rencarnation (however you spell it) of Kikyo, Kikyo as a dier requst to Kieada was to have the Shikon no tama a.k.a Shikon Jewel burned with her body so that no demon would ever be able to use it for evil again hense 1000 years in the future Kagome was being born at the same time of the burning and/or death of Kikyo.
see resemblance (no dip)

SHe is put under a curse by a dark priestess. She blackens the jewel that nikaku gives her and the few shards that kagome has go inside possesing her to do the duty of killing inuyasha