Thank you for your query.
You mentioined that you maintain a good oral hygiene
of your daughter which rules out gum disease
. However, ulcers in the mouth can bleed. Just ensure that there are no fresh ulcers in the mouth. You also need to ensure that your daughter has not bitten her tongue while sleeping which may be bleeding. In case there is no evident cause of bleeding then an examination by a doctor will be the best course to know the reason.
I hope I have answered your query to your satisfaction.
Wishing your daughter all the best.