Hi hi, her eye is red in Heavensward because Bahamut finally woke up after his slumber since the Calamity due to her unlocking Dreadwyrm Trance.

He started stirring within her soul around the end of ARR. The dragon's roar at the end of the credits (assuming it was one of the First Brood or Midgardsormr) was the catalyst. When she finally meets Midgardsormr himself during the events of the Keeper of the Lake, he comments vaguely comments that his son (he's addressing Viria as his own child) will awaken soon.