Stumped

So does this sound familiar to anyone?

Little Fish wakes up each morning bright eyed and bushy tailed, raring to go, and wolfs down a slice of toast. She'll take a water bolus no problem, and whizz around in her chair for a while. As the morning goes on she gets a bit tireder, until by lunch time she's definitely droopy. She'll ask for lunch but only nibble at it, eat the bare minimum then forget she's got the rest. And fuss if anything is put through her tube. Then, just as I am planning something nice for tea, she will throw up everything she's eaten for lunch and be fairly miserable until bedtime.

I've tried more water and less water. She's had a drip at hospital and a slow dioralyte feed at home. It helps in that she's producing urine and not puking coffee grounds, but she's still gagging and retching by about 3PM, bringing back her undigested lunch and not wanting anything for tea. She throws a fever at bedtime which comes down as quickly as it goes up, whether or not she has calpol (just throws it up if we give it, so we don't).

She's eaten half a roll today. A slice of toast yesterday. A yoghurt the day before. And a piece of bread the day before that. Hospital say her bloods and urine tests are "within normal levels", she's showing no signs of infection, not dehydrated any more, nothing is showing up as a problem anywhere.

So I'm stumped - it doesn't feel like a lurgy to me, but if it isn't, what is it?
Tia