Given Sassy's age, I am concerned by her sudden loss of appetite. The most common cause of decreased appetite in a cat of her age is kidney disease or failure. Kidney failure in cats is generally a chronic, slowly developing problem, but it can also manifest as a sudden change. Things that you can look for that may represent a more long term problem include weight loss, occasional vomiting, and increased thirst. Kidney disease can be picked up on routine blood work at the vet.
Another possible cause of decreased appetite in an older cat is hyperthyroidism. Hyperthyroidism is also associated with weight loss, sometimes increased appetite, but it can make them feel sick and not want to eat. Again, this is detected on blood work at the vet.
Regarding the blood/mucus in the stool- this sounds like colitis (inflammation of the colon). Causes of colitis include bacterial overgrowth/imbalance, food sensitivity, dietary indiscretion (getting into something she shouldn't have), parasites, and lastly a tumor affecting the colon.
Because Sassy is considered a senior cat at this point, my best recommendation is to have her evaluated by her regular veterinarian. They will do a physical exam including assessing her hydration level and abdominal comfort. Based on their exam and her history they may also recommend blood work or imaging. Does Sassy have a primary care veterinarian to go see?