OK, recently I've upped my activity and am making a point of getting cold sometimes, to keep my immune system "alert" (if it's used to the cold, it will, theoretically, fight off flus and colds better than if I mollycoddle it and keep it at exactly 36C all the time). This means I need more kcal. I know I need more, because I feel hungry, but have no particular "craving" or interest in food, which is usually my body asking for an energy source.

But I can't eat. I literally feel sick when I put even a nibble in. And it doesn't seem to be psychological: I can chew and swallow fine, but, about 3-5 minutes in, when the stomach has started to break it down properly, I feel sick.

It doesn't seem to be stomach size, either, as I haven't eaten recently enough to still be full, besides a few bites of this and that, trying to fuel myself. It's literally as if my body's got a "split mind": one side wants energy, the other has a kcal limit. /:

My body seems to have "appetite cut off points":
1300kcal if the day is warm and I'm inactive, which I can boost to 1500kcal without issue.
1500kcal if the day is cold or I'm active, which I can boost to 1800kcal, no probs.
But much over 1800kcal and my body starts acting up.