GF has a message on her phone, from a girl. I ask who she is,

>>18478619
"Girl" is probably a guy that your gf put down with a fake female name in her phone's contact list.

>>18478966
Bingo.

>>18478966
Could be so
Really confront her about it and make sure she's aware it worries you

>>18478619
beat her up until she tells you

>>18478619
If you keep asking about she'll go incognito mode. Best to play it cool and look through her phone when possible. If she thinks you're onto her she'll keep doing it but change up tactics.

If you're this suspicious and paranoid, break up with her. You clearly don't trust her.