I can totally relate to this. Mine was particularly severe during the depths of my anorexia which brought about all my problems. it still returns when I push my body hard, but not quite to the same extent. It was a feeling that nothing mattered- but thats not to say a happy or cavalier feeling. A feeling that there was the world and you. You could be with people, but you weren't really with them. you weren't even with yourself. The things you'd been interested in in the past meant nothing and you could barely sit down for a few minutes to focus on anything. i couldn't stand TV for more than a minute.
It is a horrible sensation- whilst feeling this detatchment i sent someone a very stupid letter based on a deluded paranoia and really messed up. Thinking about it now it seems so ridiculous I cringe beyond belief. How could i have done such a strange thing? Well- thats just proof of how powerful the mindset of depression and anxiety is. Its always felt to me that i can feel the inbalance causing this feeling when it comes on, perhaps you can fix it by a change in diet or activity? If not have you considered speaking to someone like an endocrinologist? I know health anxiety puts you off of health care proffessionals sometimes. If You don't want to do this, I recommend trying as hard as you can to relax. Eat some really healthy, nourishing foods. get a little light exercise or visit a nice peaceful natural place. If you can face it, see your loved ones in a calm setting, speak to them and try to remind yourself that you are you and you are loved. Best of luck.