Yeah, be sure there is not pain issue. it sounds like an evasion, to me.
I would first check for obvious pain issues, though they would likely be evident when you rode her in that direction. Then, sort out this evasion . Disallow it, get her going forward at the canter, then cease lunging for a bit. horses get really tired of lunging. Wouldn't that bore you out of your skull?
If she stops and turns to face you, put a bit of a pull on the line in the direction you want to go, put some pressure on her , and if she backs up keep asking and keep putting some tugs in the direction you want her to think and go. If she really insists on backing, even after you keep tugging in the right direection, then put pressure on and back her vigorously. Make her choice to back up be a hard place to be. When she comes to a stop, ease up and ask again, Go right!