Eeks. So, are you in a round pen? I have found Monty Roberts a great resource for stuff - you can check out a lot of resources on his site (google Monty Roberts). He is NOT a proponent to lunging without side reins. The logic being that the pressure torques their head in slightly, throws the body position so that there is no bend, and causes discomfort. Maybe your girl has a vertebrae out in the upper neck area?? So, another question would be, how are you lunging her? What equipment are you using? Have you ever long-lined? I used to lunge my horses with side reins for many years and then just learned to long-line - OMG it is a dream. You can establish outside contact really well! But, you must learn how to do it and not get hurt, so practice on a quiet gentle horse, get the feel, and try it out for reals.
Also, when horses get a little crazy - just stay unemotional about the whole thing. I am training a 3 year old Holsteiner who likes to be dramatic. If he kicks up his heels and jumps the moon, I just go, oh well and move on to the next thing. Horses are "in the moment" so it makes sense for us to also be "in the moment." Try to be even with emotions, be cautious without being scared, be infatic without being angry or bossy, and be forgiving. Good luck, and know she is teaching you something!