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Often at this age a dog contracts parasites again. This can make it hard for a dog to hold their bowels and once that happens, then end up breaking house training. So I would have her dewormed first. Common dewormers are Panacur, which is a prescription product so you need to obtain it from your Vet and it works on roundworms and whipworms. Wormers labeled as Pyrantel Pamoate are the best over the counter medication for roundworms. Drontal plus is also a prescription product and is used for the treatment of Tapeworms, though dewormers with praziquantel work well against tapeworms.
You will need to deworm at least twice 2 weeks apart to get both the live worms and any hatching eggs. YOu may have to retrain her again but once she is worm free, she should be able to hold her bowels again and train easily again. If you want to be sure, you can take a fecal sample to your vet and have it tested. Most vets will do this for you and confirm if it is parasites or not.
Other causes could be a protozoan infection such as coccidia or giardia. Here is some information on those conditions.
The vet should test for these when he checks the fecal sample as well.
If he is parasite free, then you will need to retrain her again. Usually using the method that worked when a dog was a pup is your best way of retraining, though crate training is the method I usually recommend.
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