If your normally fearless puppy suddenly turns shy one day, don't panic. It is normal for puppies to go through several fear periods as their brains develop. The first generally occurs somewhere around 8-12 weeks of age and another period occurs around 5 or 6 months of age. Depending on the breed and bloodlines of your dog, your dog may experience more or fewer fear periods. Do not panic; just let your puppy go through this phase. You may want to avoid going to the vet, training class, groomer, or new places for a week until your puppy is back to his normal behavior.
If during a fear period something frightens your dog, it imprints very strongly. So, rather than trying to work through a fear period, it might be best just to let it pass.
Adolescence starts at about 6 months of age and usually continues to 12 to 18 months of age. Adolescence is when most dogs are turned over to shelters. This is a period when puppies start testing their world and their boundaries. A previously "good" dog may become a nightmare.
Continued obedience training, maintaining structure and boundaries, patience, and skilled management are all essential practices during this phase. Management means setting up the environment so that the dog doesn't get a chance to do "naughty" things, and includes techniques like crating the dog when you cannot supervise directly.