Oh, that sounds so stressful! My first thought is it could be an ear infection. That can make it painful to nurse. I'd suggest having her doctor look her over just to see if something like that is going on.
You could also try laid-back nursing and see if it makes a difference -- if she is uncomfortable or in pain for some reason, maybe a very different position would help.
Whatever it is, I hope it passes quickly. Poor baby (and poor you, for that matter!). Let us know how it goes.
My brilliant "babies":
Butterfly - 13 yo - environmentalist
NinjaBoy - 10 yo - stuntman