Somebody’s heart is in their mouth
Definition 1 of 1
Pro Tip 1/3Intensity of Fear
The phrase highlights a very high level of fear or nervousness, not just a slight worry. 
Watching the horror movie alone at night, her heart was in her mouth.
Pro Tip 2/3Figurative Speech
Understand it's a metaphor; someone's heart isn't literally in their mouth, it's used to express strong emotions. 
When the lights went out, his heart was in his mouth.
Pro Tip 3/3Temporary State
Use it when describing a momentary feeling, usually related to a specific event or situation. 
Her heart was in her mouth as she awaited the competition results.