It sounds like kidney failure is a good possibility. Your vet can do a blood test and urinalysis to confirm it, but there isn't anything to do about it other than treat the symptoms. On the other had, it could be only a simple urinary tract infection that antibiotics will cure. She really should have the full "senior" workup done to see how all her organs are functioning at this point in her life. The point being that you can keep her (and you) as comfortable as possible for as long as possible.
That's a tough call. First thing would be have her checked out by a Vet. for any health issues that might be going on.
It sounds like she might have some kidney issues. Your probably not finding much fecal matter if she's not eating. She probably has cataracts in which the light would bother her. Try to keep fluids in her.
I used to feed my 16 yr old Chihuahua Eukanuba kitten food. This will help to keep her strength up and give her a good balanced diet of what she needs. If she can't chew it; soak it and mix some "little" favorite tid bits of food in with it. My little man pictured was almost 17 there. He got very very thin. Keep her warm.
They are tough little dogs and don't usually show pain. One day while Poquito was very weak, I had to leave and came back with the dreaded question on my mind as I had several other times in the past 2 years prior. (that he had always seemed to pull threw and come around again), but this time he couldn't, he waited for me to come home, I picked him up and held him on my chest for about 20 minutes as he was laying on my chest last July he had a heart attack, let loose and was gone in my arms. I miss him bad sometimes. I wish there was a clear cut answer, but, as I said these little guys are tough.
The best thing to do when they get older like that is to give them all your love, keep them warm, make sure they keep fluids in them and feed them a balanced diet. I don't know if this helped, but, I know how you feel. Take Care;
Amy