2) Pent-up energy.
Dogs have a certain amount of energy that needs to be released and if it is not, this will most often manifest with hyper, often out-of-control behavior. A dog that is long overdue for his daily exercise
may bark at nothing, chew things up, run in circles and find other ways to show that he's really in need of stretching his legs and blowing off steam. A Shih Tzu should be walked twice per day, for at least 20 minutes at a pace that is brisk for the dog. Be particularly aware of this during the winter
, when getting out may be more difficult. Bundle up (both you and your Tzu) and apply paw wax to protect from the cold, ice and ice melt chemicals. On stormy days, do not assume that your Shih Tzu 'self exercise'; instead, do a planned activity such as hide n' seek or play fetch in a hallway for those 20 allotted minutes.
3) Being mishandled at the groomers.
Sadly, this happens far too often. It is very common for owners to say that their Shih Tzu started acting strange after being at the groomers. And more often than not, it happens with a groomer that was known and trusted. Even good groomers can have off days where they do not keep their stress in check and accidents can occur in which owners are not notified. We'd suggest calling the groomer to ask if anything out of the ordinary occurred; do not blame but rather ask for help in helping to identify why your Shih Tzu is acting oddly after the visit. You may be told that your dog was especially nervous during a nail trim (in which case you may want to take over this task at home; electric filers can make this easy) or that your Shih Tzu was fine with a body trim but acted quite afraid when his ears were touched; in which case you may want to make changes to what sort of trim your Tzu receives. But also, if one particular body area set your dog off, this can be a red flag of a developing health issue; for example, intolerance to having the ears touched may point to an ear infection.
This all said, some dogs simple dislike certain people for reasons that will never be explained. It can be a gender issue or even a person's particular smell. A Shih Tzu may have always disliked a certain groomer and finally crossed a line in the sand where the stress of contact was so great it culminated into acting afraid or strangely afterward.
In any case, we would recommend locating a new groomer that allows you to view your Shih Tzu while being groomed. There are salons that have an owner's viewing area. You will be allowed to see your dog through a window for reassurance that he is being handled with care.