Would you describe the  quality of your sleep lately as “below average”? 
Are you on Twitter?   

Answering Yes to both of these questions would probably explain it. 

“From time to time”
As in, once a month, once a year, or every few years, or once in a decade?

If once a month, depending on the severity (and what you’re describing does sound intense), you may consider talking to a professional. Inexplicable “impending doom” that you feel “all day” is not typical.  A qualified therapist could help to identify the source of this feeling, or if there’s a pattern that triggers it.  If you keep a journal, that can help, too. 

Take care.