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Most female dogs go into heat between 6-12 months but it can be a little earlier or a little later. When a dog is in heat, the vulva swells and there is usually blood show of some sort though a dog may clean themselves enough that it isn't noticable. So you can check to see if her vulva is swollen which would signal her being in heat and able to be impregnated. If she is, you want to keep males away from her.
Now if the shih tzu was just on top of her and not humping, then that may have veen play behavior. Dogs will even hump one another in dominance which is also unrelated to breeding. However, when breeding takes place, dogs achieve a tie (stuck together) when the male ejaculates into the female. If the dogs were tied, then there ould be a good chance of her becoming pregnant.
If you witnessed a tie, you could have her spayed as soon as she comes out of heat and stop the pregnancy in that manner if you had no plans on breeding her. Otherwise, you would need to be prepared to deal with possibly taking care of any puppies if she does not care for them due to her age. Most dogs don't have problems even this young, but it is better if she doesn't breed until she is older.
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