if this girl wants to get back together with her ex, then obviously she is choosing him over you; and if that's the case, then I would let her go, because unless she really loves you, it's not worth holding onto something that's not solid.
Walk away but first confirm what you found out is true.
Give her space to figure out what she wants.
Talk to her. Communication doesn't seem to be popular in relationships.. whats the deal with that?
Do you really like her ? Then just sit back, keep you options open and continue to date her. Maybe be a bit distant, make her chase you.