An undefined feeling of impending doom is associated with both anxiety and depression. I've experienced it myself from time to time over the years. I'm not sure I can offer the best of advice--I mostly suffered through it until it passed. But it has always passed.
If I could offer any advice, though, it would be to try to ascertain the route of it and examine that. Perhaps an understanding of the source can help you to overcome the problem. For me it was more anxiety than depression. Do you experience a lot of anxiety? What situations exacerbate this? Next time you feel this unknown dread, try to consider what happened just before it started, what you were experiencing or thinking, and then what lead to those things as well. It is not a quick-fix by any means (in that regard, a distraction such as company, a movie, or engaging in a special interest might be best--but this will only be a temporary solution), but problems such as these are rarely fixed quickly and easily. At any rate, even if searching for the source of the problem does not end with you being able to eradicate it, the knowledge should be able to help you to anticipate and avoid the problem in the future.
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"A flower falls, even though we love it; and a weed grows, even though we do not love it."