Awww, sorry you are having such a difficult time, it must be very frustrating.
From what you've said, the only thing that really springs to mind is an over-active let-down, which is when your milk comes out too fast for your baby. However, if that was the case you would probably notice milk spraying when the baby comes off to cry! Maybe someone else will have some ideas. Have you spoken to your health visitor or someone about it?
Perhaps if she is less fussy at night you could try lying down to feed during the day? I had to feed Aaron lying down for several weeks, as he just wouldn't feed any other way. Thankfully I can now sit up to feed him during the day!
Anyway, here is a link that may help.
http://www.kellymom.com/bf/concerns/...e-nursing.html 

