You might want to see a doctor about that.

yep. as someone who has suffered sleep apnea for a while once your oxygen levels get so low that you can't hold your breath for 4 seconds it's time to get checked.

You might be breathing with your chest instead of your belly. Try filling your belly with air as you inhale.

This! Also, probably connected to bad posture, specially when sitting in front of a computer.

My posture when using the computer is terrible and usually when I'm distracted, I slouch and kinda slide down on the chair, that makes my back arch forward and compresses my stomach. Since I'm distracted reading or gaming or something I just notice it when I already start feeling my heart on my chest, or slight discomfort or shortness of breath.

I have health related anxiety and this has caused me lots of stress in the past (bad posture making me overly mind how heavy it is to breathe, or starting to feel my heart beat on my stomach out of nowhere and get stressed out etc). I've been focusing on sitting better when in front of the PC and correcting myself early when I slide down and the issue has gone away so quickly and in such a way that it even feels ridiculous to have been so stressed out about it. I truly believe our best friend and also our worst enemy is the self, so we need to get our feelings in control, and that means good breathing... and that can require you to better your posture :)

The timing of this post was very good because I tried doing the breathing exercise 5 mins ago and while I didn't had shortness of breath like the poster above us, it wasn't calming to me either... I then corrected my posture, redid the exercise and yes, it did relax me quite a bit :)

Good posture, mates!

Try counting to 2 instead. You may have to work up to it. Definitely don't strain yourself or you may get the opposite effect.