I accidentally saw a message where my girlfriend said she sometimes thinks about another guy. What should I do?

I have a girlfriend whom I love. By accident, I saw a private message she sent to a friend where she said she sometimes thinks about a guy in her dorm and finds him attractive. She also said she loves me and doesn’t intend to do anything with him.
It really hurt me, especially knowing she thinks about another guy while we’re together. She doesn’t know I saw the message.
What should I do? Should I tell her I saw it, or leave it alone? How should I handle my feelings without making things worse?