Just ask her

 I'm sorry that you're having these experiences, but it is one of the drawbacks of sudden, intense relationships, especially for teenagers and young adults.

If you feel that your girlfriend is cheating, I would suggest a direct approach, rather than swiping her cell phone, etc.  Just ask her, straight up. Her response to your directness will give you a good idea of where her head is at and then you'll have to make a decision:  trust, or don't trust.  When I was younger, I carried spare clothing, etc., in my vehicle because I often spent the nights with friends, and this might be her situation.

Whatever your concerns are based upon, please, try to keep in mind that strong, healthy relationships are based upon trust, honesty, mutual respect, and support.  Without those values, any relationship is going to be difficult to maintain.

Good luck to you, and best wishes.