I have panic attacks every now and then, and for me, they can be sudden or a build up.
I have them in situations where I have to speak in front of people - if it is sprung on me, I freeze up and almost pass out. If I have warning, I dwell on it and have an attack usually right beforehand - and then I calm down enough, but hardly ever make it through. I even get them if I need to make a call, to say, an insurance company, a Dr. - some people would laugh at that, but those are things I dread.
There have been times, though, as to where I will get this feeling and my heart starts beating faster and I just feel like I'm caving in. And there wasn't a reasong behind the attack at all. Those, however, are less frequent.
I have gotten better, but still need to be careful. I was diagnosed with Severe Anxiety Disorder several years ago, but it is not near as bad as what it used to be.