Rest API :Cookies

hi,
after the authentification through my mobile app i have got the cookies like this :
{Cookie: sid=de5dc0dc7731b41cfb4d6cf53d6a13c4824c5ac4d97f1090be9abccc}
but, when i set it in my get request header i have got this error : GET /api/resource/Employee HTTP/1.1" 403 -
ā†’ forbidden request !!!
any idea ?
help please
thanks in advance