How to stop suffocating feeling?

I keep getting this suffocating feeling, especially today, and it seems to be triggered by stress. I involuntarily inhale deeply and stop breathing for a few seconds. My stomach sucks in and my shoulders raise and tense up which hurts my neck. I don’t know how to stop this when it’s happening. I’m trying to be subtle about it so people around me don’t worry I’m choking or dying or something but is there anything I can do to alleviate it in the moment? I tried to practice diaphragmatic breathing earlier but that just triggered the suffocating feeling.