Dexter Protecting...I think

Question
Recently my dog seems to have gone into protective mode. He normally sleeps with me or my dad but now does not.  Now he will only sleep downstairs in the living room or hangs out in the kitchen by the back door.  His behavior is the same otherwise, still playful, still eating, but when it gets later in the evening, he does not come when called nor does he want to come to me or my father.  Normally i dont have to call him to come upstairs to go to bed with me, he just follows. Now he stays downstairs.  Last night he slept the entire evening downstairs in the living room alone. The night previous to that i brought him upstairs with me and he slept by the closed door the entire night.  The night previous to that, he slept practically on top of me in the bed, something that he never does.  A friend came over the other night, Dexter loves her. He always jumps on her and is very excited to see her.  The other night, he did not even jump when she came to the door...again it was very late in the evening.  

Dexter will be a year old in one month.  He is part schnauzer and part terrier.  He is an alert dog that barks when he needs to but is not a barker.  I walk him often and play with him constantly.  Dexter has never acted like this before.  Another strange thing about his behavior is he has resorted back to some of his puppy behaviors like sleeping under the bed, under my legs, he liked to sleep under things when he was little and has started it recently.  He wimpers often which he has never done.

Any thoughts on his behavior, please let me know.

Answer
You don't mention whether or not Dexter is neutered.  If he's not (and even if he is but it's been done recently) he may be responding to scent from a bitch in heat somewhere in your vicinity.  A male dog can smell a bitch in heat for miles and this will disrupt his normal behavior.  However, obviously any abrupt change in behavior requires a routine veterinary checkup to be sure there isn't something biological causing the problem.

Having said that, the other alternative cause which comes to mind is that Dexter has acquired some sort of fear conditioning which could either have occurred while he was upstairs during the night (which would explain his avoidance of spending the night in his accustomed places) or something that occurred outside your home during the night which might have placed him on 'alert'.  This could have been a harmless noise due to traffic, wind, etc. or something involving people or other animals.  It's impossible to know which is which but I strongly suggest you pay a visit to the veterinarian, report these recent changes in behavior (especially the whimpering!) and have a routine cbc done.  At his age it's not likely there's a physiological problem but the situation calls for this checkup.  The vet should also do a neurological exam as seizure disorders often appear in dogs around this age and they don't all involve visible physical activity.  Seizures can create sudden changes in a dog's behavior without being obvious to humans.

Regarding his reversion to puppy behavior, age 1 is the first threshold of adult maturation in the domestic dog and often one sees unusual puppy behaviors and the emergence of adult behaviors, such as guarding.  I wouldn't worry about the puppy behaviors nor would I particularly worry about the sudden independence, because both should resolve within the next few months.  However, I would be concerned with Dexter's sudden loss of enthusiasm at the appearance of a 'friend' he once greeted.  See what the veterinarian reports after his checkup and give him a few more weeks, while observing any further changes in his behavior, then report back.  If the behavior becomes worse, or if you become aware of what might have frightened him or caused fear, re-post.  if YOU have changed anything in his environment (rearranging furniture can create all sorts of superstitious behaviors), or if you have recently changed his food, these may have contributed.