Bladder Infections
Bladder infections in dogs are common and a dog with an overactive bladder is one symptom on bladder infection. A dog with a bladder infection my have penis swelling associated with his overactive bladder symptoms. A bleeding dog penis accompanied by swelling is also a symptom of bladder infection.
Mating Behavior
Arousal in male dogs is common and the arousal may look like the dog's penis is swollen. When a male dog mates with a female dog his penis will become erect and the glands at the base of the dog's penis are what help the dog "tie" to the female dog during the mating process. If a dog becomes aroused and is not participating in direct mating behavior with a female dog, the penis may appear swollen because of the glands at the base of the penis.
Usually a swollen dog penis is associated with mating. However, if your dog's penis is swollen an extended period of time then you should look for additional symptoms of bladder infection. Untreated bladder infections lead to further kidney problems that may require dog kidney dialysis.