Well, I have no idea why those attacks would come out of nowhere, that is strange.
But then again, my panic attacks come out of nowhere too.
Maybe there is some problem in the back of your mind that you have not confronted yet?
As for the symptoms, like the nausea and feeling like you are going to get sick, that is usually from hyperventilation.
Do you have irregular breathing?
Maybe you could just use some fresh air or take a walk around the block.
Also, in opposite of that, is to take a paper bag, or just cup your hands over your mouth and nose and take a few deep breaths.
That will usually calm most people down quickly.
Has there been a change in your diet, or maybe the meds are not as effective as they once were?
I just hope you can find some relief soon, anxiety is a scary thing.