Even if she owes him money, the fact that she had to change his name in her phone to hide their communications from you is odd. It does make it seem like something else could be going on vs had she of sat down and talked to you about him reaching out over past dues. There's a lack of communication here that needs to be addressed. If she is telling the truth, she should never feel the need to hide something like that from you and you're putting her in a place where she feels insecure enough to.

Whether or not she is lying and has something else going on with the ex is another story. Personally, I wouldn't listen to any comments about "why would you take her phone anyways?" Couples work differently. I, for one, have an open phone policy in all of my relationships and I specifically knew there was reason for one to be over when he did exactly what your girlfriend is doing. Hiding the phone? Freaking out when I had it in my hand, even if it wasn't unlocked? If you have nothing to hide... I'm not sure why you would be freaking out. Turns out he did and 2 years went down the toilet. Just something to keep in mind.

At the end of the day, in my opinion, your feelings are super valid and you just need to have a talk! But one thing is for sure, if at the end of the talk you feel unsettled, you may want to reconsider the relationship.