hey there, I know the feeling of feeling low without any reason at all, actually this happens to me when I eat bad carbs of dairy. But vlc / high fat cures it, so I agree it can be dietry related.

I know some people say perhaps you are short of something, try having seaweed/kelp some succeeded and noted a change in the way they feel if they had it.

I am not really sure what is your diet like, i.e. fat/protein and calories etc, one needs to experiment to find what's good for them.