[yorkietalkjilly]

When a dog's behavior changes and you can't directly relate it to a current life-altering event or stress you know the dog has undergone, I would have the vet check the dog over. Dogs in pain or experiencing illness or injury can nip from the stress of what they are dealing with.

If you rule out a medical problem, then the dog is likely trying to assume too much of a pack leader role, trying to become dominant or expressing frustration, fear, anxiety from its current lifestyle. A whole program of Nothing In Life Is Free, getting him busy with a job of learning a good obedience program for 5 minutes a day, 3x daily, teaching or re-teaching the dog to perform basic commands for praise, reward, bonding and self-confidence, adding physical and mental stimulation challenges to his daily life and daily enriching his life with added socializing and vigorous exercise, even agility, can all work together to reshape a dog's bad behavior and slowly watch any new bad habits or misbehaviors begin to fall away. Being pack animals, dogs are genetically programmed to respond to an hierarchy in his family and if the dog isn't ill, likely a lack of strong leadership or consistent boundaries have stressed him into trying to become the leader he lacks, though no pet dog is really happy in this role.

If the dog is a big powerful breed, this behavior has to be addressed at once to prevent a possible tragedy. If you are not someone who is comfortable becoming a strong, confident but loving pack leader, perhaps an outside behaviorist needs to be called in to teach you how ASAP.

But the first thing is to have a vet check and muzzle the dog around children, the elderly and smaller pets until he is under your firm control.

P. S. In re-reading your OP it sounds as if the nipper isn't the pitbull but even so, all of the remedies are the same except perhaps you don't need a muzzle as a small dog's biting rarely leads to a tragedy.