That's very strange. She could have gotten a sore on her neck that made her start scratching it and now it is a lot more than a sore. It probably just hurts so she is scratching it right now. Just make sure to keep an eye on it, especially for infection. Don't forget to keep me updated. :)
We didn't put any treatment in with our chickens till they actually had lice. We use Seven powder and put it in the coop, along with dusting our chickens twice a year.
Do you have a place that you can move her? I would recommend moving her inside and away from the flock to avoid getting pecked. Give her some electrolytes and check on her often. Do you think she would be okay with a gauze pad?