I have had this for as long as I can remember, and for me it is anxiety. I have some triggers - (I have learned that alcohol, lack of sleep, too much caffeine or generally not taking care of myself properly can trigger it, so I avoid those especially if I am already feeling angst. Being ill or premenstrual can bring it on, too, so at these times I am extra gentle with myself) - but at other times it is seemingly random - generalised anxiety is the term used by the medical profession, I believe.
It did spill over into some more serious mental health issues for me for a while, but I am better now, yet still experience this. I have come to accept it is part of me, really, but lots of little things help.
Mindfulness has helped me a lot. It doesn't stop the feeling from coming on, but it seems to help me keep it under control, so that it only lasts minutes or hours and doesn't (often) spiral into days.
When I do have longer periods of feeling like this, I try to be kind to myself and just accept I am having one of my 'dark spells'.
Exercise also helps massively, although it is a real effort on days when i feel like this.