is usually because of adenoid enlargement.
Nose, conditions and filters the air we breath in by various mechanisms. This will be absent in mouth breathers and they will be prone for frequent respiratory infections.
And dark circles
around the eyes suggests allergy.
Its better to have a consultation with your Pediatrician
and ENT specialist
. If she gets very frequent episodes, they might suggest removal of adenoids. If not severe, they will suggest medications.
For allergy medications are available in form of nasal sprays that will prevent such attacks.
About her teeth grinding, some of the proposed causes are teeth malalignment, Iron deficiency anemia
, pain in ears/ teeth, some stress at home or school. Look for these causes and if nothing contributory, don`t worry about this. Kids outgrow this as they grow.
Hope my answer helps you.