Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry that Mattie isn't feeling well.
If she had only been "off" for a day or two, then I might not be as worried; dogs will have off days now and again but then bounce right back to their normal selves. But since it's been three days since she's eaten on her own and is vomiting after she drinks and she's also lethargic, I share your concern that she's may be dehydrated and that something else may be going on with her.
Given her age, she could have issues with her kidneys or liver or pancreas or even spleen such that the longer she waits to be diagnosed and treated, the more serious her situation might become.
These older dogs simply don't have the reserves of their younger counterparts and it doesn't take much for them to deteriorate very quickly. Even forcing fluids into them (if they aren't vomiting) is often not enough to keep up with their fluid requirements.
I typically would possibly suggest otc medication such as Pepcid AC but I don't think it will be an effective solution to her problem, unfortunately.
If you could have her seen today, that would be my recommendation.
I hope this helps and that she'll be ok. Deb