You've heard it time and time again, SHOW DON'T TELL. If you have crit partners you've probably gone cross-eyed from reading it in your ms at one time or another. But you're past that now. You've worked hard on your prose, you're showing all over the place. Or are you?
Using internal physical reactions is a quick way to show a character's emotions. You've seen sentences like these:
My heart raced with fear.
Nervousness twisted her stomach.
These sentences seem, on the surface, that they're showing but in reality, they're telling. Why? Because it tells us what emotion the character is feeling. Fear, in the first sentence, and Nervousness in the second. Chances are, if the physical reaction is appropriate to the scene, that the naming of the emotion is simply excess information. This is sometimes called tagging your emotions and it's usually unnecessary.
In this case, that extra info creates a distance between the reader and the character. In a tense or emotional situation, the reader should be right there with the character, experiencing and connecting to everything the character feels. When something happens that causes your character's heart to pound your reader feels it, when you add in "with fear" you push your reader back a step because they're forced to process an external observation.
Think about it. When you're in the middle of a scary situation, you might notice your heart is pounding but do you actually think - hey my heart is pounding because I'm afraid? No. You just feel afraid.
I work with the rule of thumb that unless a character is experiencing an emotion that is unexpected (like, rather than fear, a character's heart pounds with excitement at being chased by an axe murderer) there's no need to name it. If you've done a good job at creating your character and revealing what makes them them to the reader, they will know what your character is feeling. And even more than that, they will feel a part of that character's experience.
Trust your reader! You don't have to explain everything to them.