What causes the feeling of “impending doom”?

I’ve been dealing with GAD and panic disorder for years now. It’s been a journey with many ups and downs, but I ’ve been having increased symptoms lately.

Quite often I’ve been experiencing a sudden feeling of impending doom out of nowhere. The feeling that something is very, very wrong. It literally comes out of nowhere. And there literally nothing wrong that I can identify.

At this point, I’m familiar with the feeling so it rarely leads to a full blown panic attack. But it still feels SO uncomfortable and sensitizes my body which can lead to prolonged feelings of heightened anxiety.

Can anyone else relate? Or know why this feeling comes on out of nowhere? Has anyone found an effective way to cope when this happens?