I can totally relate to this - I'm all or nothing when it comes to food.

I often feel nauseous and headachey for a day or so before a session (and sometimes after for a couple of days) and its totally anxiety based. All I want to do is lie quietly in an empty room in peace (which doesn't happen too often as I have young children!)

Sometimes fresh air and a distraction helps too, even though I usually don't feel like going anywhere, once I'm out there it can lessen the symptoms.