13 year old dog is lethargic, won't move, has diarrhea- what do I do?

You should call any vet the is surrounding your town, do you have a animal shelter there? Make sure the dog is hydrated and take her outside, she might just have a belley ache from not eating a few days and then eating food with such a substance. Your best bet would be to feed the dog chicken (boiled easier to shred) with chicken broth. Hope she gets to feeling better :)