Maybe someone hurt you and broke your trust in the past?
Its a hard thing to give someone your trust after you've been hurt.
My only advice is, not trusting her and getting upset only hurts you, it does nothing to solve the problem.
Also, it takes up a lot of your time and engery worry about something that may or maynot be happening.
You could try telling your self a little phrase each time you feel that way,
Give me the strength to accept the things I cannot change,
the courage to change the things I can and
the wisdom to know the difference.
Try doing something you enjoy when you feel that way, don't waste your life worrying about something that you can't control. Its only hurting you when you do that, be nice to yourself.