What is ACTUALLY happening is like a cascade of anxiety piled on top of anxiety. So you feel anxious - and your body triggers as part of that a physical response designed to grab your attention. But if you don’t realise what is going on, or notice, it ramps up that response - and then sometimes when you do notice it, instead of realising it is anxiety, it feels SO PHYSICAL that it triggers fear of what is going on or what might happen. And of course, that fear makes the anxiety response worse, so the symptoms get stronger - and it can spiral up and up until it gets really full-on.
Panic attacks can look really dramatic and it's not unusual for people to call an ambulance, because they look so awful. But actually they are nothing like as bad as they seem. A lot of the weird stuff you feel is because your breathing goes so haywire, and the levels of important things like oxygen and carbon dioxide in your blood change. Once you calm your breathing down and drop that anxiety level everything will go back to normal.