I don't think 3 weeks is a short time to feel betrayed at all , it all depends on the level of trust you developed towards her. In my opinion you should ignore  knowing that fact  after all you said yourself that  you didn't have a chance to  define the relationship between you two.  That basically means  your commitment to each other  isn't agreed between you two. If you feel something strange from her tell her talk to her about it and ask her if she is sure about your relationship maybe getting in touch with the x means she hasn't gotten over him. Let her finish her business there and it will only strengthen your relationship with her. It will show her you trust her and if you intervene she might tend to get away from you.