12 weeks, new relationship, not over the moon in love, should have been called " dating" not relationship at that point.
You can't wonder about cheating, it will happen or it won't pending if you are with the right person or not.
There should be no secrets at 6 months, tell her to continue openly face-booking her friends and texting, that you trust her.
Without trust there is no relationship.
If she does the wrong thing? Well move on, hold your head high, smile at her and tell her she wasn't worth your time and that there is someone better out there for you, so thank you now I can move on and find her.