I don't think you should volunteer. I don't think you are ready for that. I think you should stop making yourself go out, stop trying to be 'normal' and accept that you're not. Not at the moment, though it could change at some point.
I don't think you ought to try and engage with other people ifyou don't enjoy it or it makes you feel worse. I think stop, retreat, and reassess - see how it feels not doing all that. not trying to fit in and be normal.
It'll at least give you some perspective.
The psychosomatic symptoms you are having will worry you, that's normal, but they aren't going to hurt you because they aren't caused by anything physiological. They are stress making you notice every little thing, sad hormones being released, reactions to your unhappiness.
This is alright - it's just your body reacting to your mind. The one thing that will help you the most, IMO and I might be wrong, but that thing is accepting that your life is shit at the moment.
You have to accept it and then you have to deal with it. I don't mean change anything. I mean, look at yourself and think, realistically, Ok, this is something I can do, and this is something I can't. Break down how every thing you do makes you feel, if it is too hard, then stop.
Be like a baby again. Except this time you are the mummy, as well, and you get to choose what you do. Put on a sweater if you're cold. Have a packet of crisps if you fancy it. Listen to your body rather than freaking out about it. The reality is, that you are alright, you're human, you're alive, you're not sick. Everything works as it should (well mostly I guess) 
Don't be afraid of these feelings. That's all they are. They can't climb out and strangle you. They re just feelings, in your head and they are part of you, and if you face them head on, and say 'I am not afraid of you', they will fuck off. Might take a short while, but they will.
it's to do with adrenalin and fight or flight...I found avoiding caffeine was a big stabiliser for me, too. Any toxins like that - they send your moods all over the place and that's not helpful when you just need to be calm.