Is it normal for 50 days old shih tzu to have mild occasional shivering on legs

Hi Amrutha. Shivering especially in pups is quite common. It happens mainly due to cold(Note that Shih Tzu breed is more prone for this) . But similar reaction is also shown when pups are scared or going through some sort of stress or pain. Do supervise him. If shivering becomes frequent, get him checked thoroughly.