I'd make sure you aren't looking at her eye and concentrate on her feet when she does this. I also wouldn't stand square with both my shoulders facing her. I notice that my mare does little half rears on the lunge when we are "discussing" things she'd rather not do. She seems to always put herself in a position where she is in front of me to pull this antic. As long as you have completely ruled out pain, I would take this as a form of protest and do anything to get her moving forward. I don't react in anyway to Frida rearing. If I can, I turn into the lunge line and ask her to move forward while pointing. If she is planted, I ask her to yield her hindquarters around me in a small circle. If she doesn't do that, we back up until she is dying to go forward. It's just dangerous to let them think that rearing can get them out of things, as it very easily can if you let it intimidate you. The only other reason I can think of for rearing is confusion, as my horse did in the beginning, but if you are clear the problem doesn't last for long.