Well, I showed and trained Shelties for years. Love them. At her age, agree that she needs a Vet check up. I would be worrying about renal problems. The weight loss isn't good. I'm dealing with the same issues with our 12 year old Greyhound now. Just took him in and he is having renal problems. I put him on the ID food for now. I would definitely be using a Senior, low protein food. Maybe adding some low sodium chicken broth to her food might help and/or the chicken and rice mixture.
I would definitely get her in for a check up.
good luck
I think the real problem is she doesn't want to eat because she doesn't feel well. The reason for this could be any number of things, from irritable bowel, to liver or kidney problems, or it could be some chronic pain (maybe her joints?) as pain can often put a dog off eating. She's not wanting to eat because she doesn't feel so good.
I think she needs to be checked out. Ask the vet to do a full senior blood panel for her, to see if there are any problems or imbalances at all going on.
If all comes up OK then she may have aches in her joints. That will need some support.
Good luck, and I do hope she is feeling better soon.
Oh by the way I never knew a dog who could resist a wholesome chicken broth made with chicken breast, carrot, and peas.....(no salt and certainly no onions)