putting my finger on it

In a funny mood today, can't quite put my finger on it, but then again you can't generally put your finger on a mood or feeling can you?
You can feel the results, the tension, shaking, tears or feeling sick, but the mood itself isn't truly touchable. Like air its all around you can feel it moving when the wind blows, it can feel cool or warm but you can't actually touch it. Put your finger on it.
It can be very frustrating when you feel something but aren't sure what it is or why, like trying to grab a handful of air !
When I'm having one of these days I find it best not to overthink it, or to think about it at all, I just try to accept I'm having one of "those days", and allow myself to rest and relax. There's no point in overthinking or worrying about these things, as that point I cannot change the outcome or feelings at that moment I can only look after myself.
The hardest part is the result of these feelings or moods, usually it means I struggle to get anything done, I can't concentrate, or motivate myself to do much at all, food has become one of my bigger struggles, I just can't face eating, I don't want anything to eat at all, even if I do feel hungry , I can't face anything let alone making anything.
I feel very shut down, and have no desire to do anything, usually it only lasts a day or so, but I can feel the effects for a few more days, I've learnt the effects of not eating enough on my system especially when doing my Zumba classes, the tiredness making me doubt my ability to teach and whether I'm doing a good enough job.
It would be nice if I could find an answer as to why, because then I would be able to put things in place to maybe prevent it recurring, make changes to help avoid things, or at least cope better when it does occur.
I hate the feeling, its a weird mixture of sad, lonely, tired, lost and stuck, like moving in slow motion or even not moving at all, time can pass without me doing much at all. Its frustrating having stuff to do and not being able to connect the dots , get my brain working. Its hard to explain and I know some people cannot understand it, which can make things harder on me or anyone struggling with something.
For now I'll continue to deal with the effects as best I can while keeping an eye out for the why. Then maybe I'll be finally able to put my finger on it.