Actually, it is very easy to get an A on every test, homework, etc, and still fail a class (the joy of bell curves).
But that aside, did you keep your homework and tests? If so, you can add up everything yourself and if your calculations don't match hers, first ask her to explain how you got such a low grade and, if that doesn't reveal where the shenanigans are, involve a school official (Principal, Vice, etc). With documentation of what you earned, you should be able to get a better grade. Fight!
I've had to do that myself. Went from a D- to a B+ just by asking the teacher to show me how I had such a low grade (she never did show me, which implies to me that she was full of it).