Hello there,
I agree with opus88's comments. Your kitty is sick and needs to see a vet asap.
Eight years old is not old for a cat. Not eating is a serious issue. She's not
having bowl movements because she is not eating. Three days is a long
time not to eat. Is she drinking water at least. Anyway, please make an
appointment with your vet and bring your furbaby in. I wish you and your
furbaby the best. Don't delay. She's counting on you. Eve
your kitty really needs to see a Vet asap.....just a few days of a drastic reduction in calories or not eat brings about a condition called fatty liver disease or hepatic lipidosis and this causes rapid organ failure from which there is very poor chances of survival...
http://healthypets.mercola.com/sites/healthypets/archive/2011/12/13/feline-hepatic-lipidosis.aspx