Most Helpful Girl
It sounds like she doesn't like either of you. She is just having fun and if one of you says no then she will go to the other. It also sounds like she enjoys the attention. Just confront her by asking her straight out. I have a feeling she will lie so consider if you see yourselves working out in the future or whether she will always be the same.
If you think she will be the same it us recommended you drop it because all it will bring you is further heartache and trouble.
Most Helpful Guy
Well she's an Ex for a reason. I would suggest not getting back with her or all those reasons will become apparent again.