The good thing here is that it could be nothing. The bad thing here is that it could be a matter of a few things. She could be having an affair...the worst case scenario, or she may want to feel more independent with her friends. She may be going through some type of stress that she is not telling you about. I have not had this happen to me personally but I would change your password on your phone to keep things fair. I really think that this may bring some things to light. This would not be done to be mean just to put things on a more even field. It is worth a try along with asking her what she is feeling about your relationship. Good luck and I hope things get sorted out for you.