If she's a lost dog you've found, then she will probably have been out a while trying to fend for herself, eating all sorts of things, raiding trash, etc. She could have eaten bones, or plastic wrappings, string....any kinds of things. It is possible she could have a blockage in her gut. On the other hand, she could have been starved, so she hasn't had enough food to make her poop. The smelly urine could be a bladder infection, or dehydration, or both, or it could be something worse, like kidney problems.
A cruel and sad possibility is that she may have been dumped by "owners" (!) who saw she was sick and didn't want to pay vets' fees!
Anyway, the best thing is to take her to a vet if you possibly can. At the least they can check her for a microchip, so her owners can be found. She may need antibiotics, and a thorough check-up.
It depends how you feel about this dog. If they find no microchip, would you be willing to give her a home? If not, you need to find out what you do about lost and found pets in your area. Or call the local animal control or shelter for advice.
Whatever you decide to do, do it quickly, as dehydration alone can be very dangerous for a dog.
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