... What? The end of the song already?

"...the end of the song arrives and you are wondering how you got there?"

This may indicate that you've lost focus on your performance.  This is, in general, not productive practice.  To play any instrument well, one must be focused.   To overcome the situation (lack of attention/focus) you can focus on some aspect of the work (i.e., intonation, phrasing,and dynamics).

I would suggest that you never let your mind wander while you practice.  Put the same focus into your practice as you would into a live performance.  A little thoughtful practice is more effective than to go through motions while one's mind is somewhere else.

No... What I am referring to is that you are so focused on what you are doing at the theremin that you feel that you've been sucked into a black hole almost... i am clearly aware of what and how i am playing. It's not inattention or carelessness. It's more like i am into the zone and all around me is all fuzzy. All I hear is the melodies, what i am playing and all contained in one bubble.

double post, sorry!

This has happened to me as well. Also, sometimes I am so concentrated that my vision starts to become blurry. That scares me a bit so I try to concentrate a bit less when I notice that happening!