You know freakinconfused...that's actually my biggest fear.
My ex and I broke up a month ago...after 3 years. Like you, it was completely out of the blue. We had made plans to move in together after graduation to go to grad school together. It was...entirely out of the blue.
We kept in touch a week after the breakup, then I (like you) went through her phone while I was at her place and found out she's been talking to some guy that I was originally suspicious of. I asked. She denied. I got angry. I left.
...she hasn't contacted me since. Although, I do hear that "that guy" has been seen leaving her place in the morning by a mutual friend. I am shocked as she isn't that type of a girl. She has the reputation of a saint. I was her first everything. But the signs are all there...he's always with her, he's talking to her always, he's seen leaving her place often, she's seen driving his car. All this...a week after we break up.
Granted, I haven't spoken to her in 4 weeks. As of right now, I have...no clue what's going on. I'm just doing my own thing...but she's always on my mind. Always.