The $10,000 question, that is! The first panic attacks I had were like that, and often the kind of situation, like what Wanderlust describes -- with nothing to be panicking about(!) I'd actually say ... maybe about half(?) of mine are that way. Plenty, at any rate.

The only other kind of situation I find myself getting panic attack symptoms is related to going too long without eating (not on purpose so much as "oh, I'll just do this one more thing then eat" kind of delay). It does not sound like that's what's going on in the situation you describe, but as we talk about panic symptoms, it might be worth mentioning. My mind goes in total freak out mode just on account of the physiological situation.

Hope you can get it figured out, because it sounds awful!