It would definitely be advisable to contact a veterinarian. While I wouldn't be concerned if it only happened once, repeated instances, especially over a short period of time, could be a sign of something serious. Use your best judgment. You know your dog best. And don't hesitate to contact a vet just to be sure everything is OK with your dog. For more information, see the links below.
I own a Shih Tzu, and they seem to be notorious for coprophagia (eating their own poo). It is possible that your Shih Tzu has eaten some poo and then is throwing it up. This is the case for mine! If this is the case for yours, there are certain products that can be added to the dog's food to make the poo "unpalatable" to the dog. (Although, as humans, we would think it to be unpalatable in the first place) There could very well be other, more serious, medically related reasons for a Shih Tzu throwing up, and a veterinarian would be able to offer more help.
It could be because its sick...
Go to the Vet!
yes yes
Yes.... Many dogs are calm and mild mannered and never throw up. I recommend you immediately take your dog to the veterinarian.
diarea
Because it doesn't belong in them.
Your dog is very sick; take him or her to the vet ASAP.
Shaking the dog won't do any good. The American Red Cross offers pet first aid classes which will help you to deal with an emergency for a dog.
This dog sounds sick. Please take this dog to a vet.
He may be sick. You should call your vet for advice.
5 min.
Uhh... yeah I would take your dog to the vet...
If your dog has been throwing up and has had diarhea for a few days, you need to contact your vet immediately, or get the dog to a vet immediately.
Call your vet for advice.
A shaking and retreating dog could be frightened or in pain.