system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What cause that "heart throb"/"tighten up lung" feeling when you are extremely sad/ heart broken?

First hand experience on Monday and still going through it from an out of the blue break up. 



Edit: thanks guys for not only sharing answers but also stories and advices. Sadly the thread is locked so I cant reply to some of you. 
"To day is gonna be a great day and you know why!!!? Cause EVERYDAY is a GREAT day!!"-MarkE Miller

EDIT2: just checked inbox and thanks again for the love through pm guys!
gpt
Effectively your body thinks that you are in fight or flight mode because it doesn't have a great mechanism to deal with heartbreak. Your autonomic nervous system takes over for your body (essentially autopilot) and tells the rest of your body "HOLY SHIT SOMETHING BAD IS HAPPENING WE NEED TO BE WORKING DOUBLE TIME TO RUN OR FIGHT OUR WAY OUT" and then your heart pumps faster and that starts hurting your chest. The shortness of breath is for a similar reason but it's because your body is trying to breathe a lot more than it needs to.