You may be reading into this more than you should. It might not be her neighbour that is distracting her. She may be busy doing other things. Is she studying for exams at the moment? What else could be taking up her time where she can't be texting as much? Best to confront her about this in a calm and adult manner. Just say you are concerned that she seems a bit distant and cold and ask her if everything is ok. Don't have a go at her for texting one word answers as this will get her back up and don't try acusing her of cheating! Hope all goes well xx