Whether or not your girlfriend is cheating, something in this relationship has changed. Your girlfriend is distancing. It could be that she has been feeling crowded by your need to always know where she is. It could be that she is trying to help you get used to the idea that she can have interests and friends that don’t include you while still being your loving partner. Or it could be that she is drawn elsewhere.
The only way you will know is to have a calm discussion with her about where your relationship is going. Tell her that you have noticed that she is less willing to share information with you and that it makes you panic. Apologize if you respond by becoming an interrogator. Ask her what she thinks the two of you ought to do to quiet your fears and at the same time to give her enough freedom in the relationship.
If it’s too hard for the two of you to come to an agreement, it may be that a therapist could help you hear each other and reassure each other. Without trust, a relationship simply can’t work for the long run. You are right to be concerned. I hope you and your girlfriend can work this through.
I wish you well.