My girlfriend is keeping slot from me ?

My girlfriend is keepin alot from me and it's all to do with her ex she goes to her exs house and when we are all out she insists on calling for him she has been around to his house quite a few times but she has only told me she has been around once and that she didn't want to and now she is keeping it from me and everytime I ask her what's goin on she says nothing and goes off in a storm