I agree with Jay and Sunkacola, that when I feel that way there is usually some trigger. I am highly sensitive and things trigger me than perhaps others wouldn't even notice, but there is a trigger for me, either external or internal.
On the other hand if a person has an actual anxiety disorder, then anxiety attacks can come pretty much at random, with no apparent trigger whatever. I know this because I have that issue.
I am a very observant person when it comes to how things affect me (have had to become so to manage my FM and other issues) and usually I can trace back to exactly, or at least possibly, what triggered it, and sometimes that trigger was actually something that happened yesterday or days ago.
But sometimes it just hits me Bam in the middle of an ordinary day while I am home and not doing or thinking anything that would set it off, and I can't trace it back to anything.
Which of course doesn't mean there's no trigger at all........ I suppose it still could have been something I didn't notice.