Most Helpful Girl
It sounds like she might not quite be over you. She may be acting fun, mischievous and indifferent to you to make it look like she's moved on. But at the same time she may still be feeling angry about the break up which is when she's mean to you. I can't explain the backstabbing of the mutual friend though unless she thinks there's something going on between the two of you. I know the above because I imagine that's how I would act with by ex BF if I were to talk to him (we broke up 3weeks ago) and that's why I know I'm not ready to speak to him yet.