I agree with the above advices to reassure her and trust you for guidance, and to get her over her fear, BUT, if she is using the popping up and backing as an evasion to not do something that makes her uneasy, that behavior in itself is dangerous and unacceptable. Don't get after her for being nervous, but DO get after her if she goes up. If she pops up, send her backwards hard and fast, about a dozen steps or so. Then offer to let her stop and stand, and think about it. Then return to the point you were at before and continue working. If she decides that her reaction isn't worth yours, that particular behavior will stop. I had a very nervous Saddlebred that had some very rough handling before I got him, and this was his reaction to anything that made him nervous (which was everything). It didn't take him long to realize that while being nervous was okay, that particular behavior WAS NOT. When he went blind, that behavior resurfaced briefly, I handled it the same way, and it stopped.
"Keep a leg on each side and your mind in the middle"