Brush your whole mouth, stay hydrated. Part of it may be keto-breath. Fasting is a driver of ketosis (fat stores being the predominant energy source in many cases), that’s a little harder to account for. Fortunately it’s not the stinky breath that’s vile-smelling but rather more chemical. You can mask it with tea or something but it’s something you’re exhaling (acetone etc) rather than an oral hygiene issue.