Good article here. As always the advice is to SEE A VET first, if you are seeing odd behaviour.

Medical conditions that can cause behavior issues in your pet

May 6, 12:47 PM Lindsey Hein - Orlando Pet Care and Training Examiner

Please take you dog to see the vet if you notice any abnormal or unusual behavior.

As a dog trainer and a veterinary assistant, I see both behavioral and medical issues everyday. Sometimes these issues can overlap or get mixed up. Many times owners believe their dog has a behavioral issue, when in reality, there is an underlying medical cause. Below is a list of some common issues that could also be caused by a medical condition. If your dog is displaying any of the following behaviors, or has any behavior problem that has occurred suddenly, please take your dog in for a thorough check up by your veterinarian before trying to address it as a training issue.

Inappropriate house soiling- This is one I see most frequently misunderstood in dogs. I always recommend that any dog with house soiling issues be checked by a veterinarian first, especially if the dog is an adult and has been reliable with house training in the past. There are several medical conditions that could cause house soiling; this site by the ASPCA has a very informative list.

Aggression- Aggression of any kind is a serious problem that needs to be addressed as quickly as possible. There are several medical conditions that should be ruled out first when dealing with aggression. Hypothyroidism is a leading medical cause of aggression and other behavioral changes in canines. Hypothyroidism can be diagnosed by a blood panel and is easy to control through medication. Other possible causes of aggression include hydrocephalus, encephalitis, head trauma, brain tumors, epilepsy and Cognitive Dysfunction Syndrome.

Sensitivity or aggression when being handled or touched- These issues may indicate that your dog is in pain somewhere. For instance a dog that snaps when you reach towards his ear may be hiding a bad ear infection. Arthritis is a common cause for older dogs to become suddenly defensive to being touched and handled.

Reluctance to perform behaviors such as sit or down; refusal to jump- These could also be signs of pain or discomfort in your dog. Any dog who is reluctant or slow to sit should be checked for hip dysplasia, a common and debilitating condition, especially in larger breeds. Refusal to jump or perform other similar behaviors could also be a sign of hip problems, as well as back, spine, or leg injuries.

These are just a few of the health problems that can be mistaken for behavior issues. Please have your dog seen by a veterinarian first if you notice any abnormal or unusual behavior.

Further links:

Medical Causes of House Soiling in Dogs
Medical Causes of Aggression in Dogs
Behavioral Issues with Thyroiditis (Dr Jean Dodds)