A 13-year-old Chihuahua still has many good years ahead of her, but the vet should examine her, and she should have blood testing if necessary. There are a lot of possibilities and an early diagnosis can make treatment easier. Sometimes if you wait for treatment, it may not even be an option. I would be more worried about her teeth than her uterus, but it should all be checked. Do not worry but do get her checked out as soon as possible.