Take her to a vet soonish like within the week. Death smell means rot - it is possible she has internal injuries with necrotic tissue (dead tissue). That is the extreme in things that could be wrong. Most likely there is an infection somewhere, be it the ears, nose throat or even sinus cavities. Unless you know she is eating something nasty... dogs do that at times. This is unusual enough to require a Vet visit. Further you will also want to have her tested for heart-worms and think about having all of her shots, distemper, rabies, etc. Have her examined for proof of being 'fixed' you do not want a surprise litter too. - - - - - - - Failing the Vet the Humane Society and even the local pound may have a program for folk taking in a stray. The SPCA (Society for the Prevention of Cruelty to Animals) around her has been very helpful to me when it comes to stray dogs and cats. Living the rural (farmland) area I come across many strays - I suppose folk dump their pets out here thinking that the rural folk need more animals to tend. Any-ways the SPCA has given me very good rates on 'Reclamation Projects' dogs and cats that I have taken in and could not afford the added expenses of veterinary bills. They have charged far less for set up stuff like shots, an overall physical examination of strays sometimes even being kind enough to bill me over a period of time for some things. Your Local pound, Veterinary Supply Store, Chapter of the SPCA all of these may offer programs for rehabilitation of strays which assist those who take in a stray in meeting the first time needs. P.S. I didn't walk in and get the help, it took a couple three found dogs and asking a lot of people "what should I do?" before I was pointed to the limited programs - you may need to call around and ask a lot of folk.

take it to a vet

Poor thing! The vet would know. I hope she feels better soon.

Maybe she has bad teeth, take her to the vet.

She might have an infection in her mouth or throat. She needs to see a vet. Free dogs are the most expensive! Good luck.

Anytime you take in an unknown dog, you should take it to the vet. She will test it for known dog diseases, such as heartworm, and the dog might even have a tag that would tell you who the owner was.