system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is actually happening when you panic and your "heart stops" for a half second and get that pain in your chest and why does it happen?
gpt
Your heart seems to stop but its actually just changing how fast its beating. This feels like an abrupt pause if it happens fast enough. This happens many times a day, you just don't notice it most of the time.

The pain that comes is a result of the fear caused by noticing and misinterpreting the change in heart rate. You involuntarily contract the muscles in your upper body, and involuntarily inhale at the same time.  Since you're trying to do two things at the same time that are impossible to do at the same time, the result is discomfort.