how to diagnose a sickness or disease? Vet care limited in my area ... trying to determine problem.

my 8 year old german shephard (female) is unwell.her blood tests have come out ok, but recently she cannot hold down or digest her food and has become thin and weak. she has been routinely dewormed over the years so i suspect that she has an internal issue. i am now making homemade chicken soup with rice to see if this helps her digest better. any suggestions on how to help her? thanks