I would put asking her at the top of my to do list, but maybe I'm just old fashioned and believe that a significant other doesn't have the right to spy on me.
If she has access to your phone she could have installed spyware to monitor your activity. Perform a factory reset (make sure to back up your photos first) to delete all applications installed on the device and return it to the state in which in shipped from the factory. Make sure to use a screen lock that you keep secret from everyone to keep it from happening again.
If that doesn't work, she probably has access to your email and/or cell phone provider account. Criminals (and potentially a privacy adverse girlfriend) often get access because people use very weak passwords and tend to use them over multiple accounts. Is you password something she could figure out, like your college mascot or pet's name? If so, change it immediately and activate 2-factor authentication of your Google account (or any other email, if available).
If you do these and she still knows your private information but won't tell you how, I would seriously question the nature of your relationship.