When you say concerning? Are you referring to messages received, or sent.
I'd suggest if it is the latter, you talk to your child, nothing worse to a child is a parent spying on them, expect a backlash either way.
If it's the former, you still need to talk to her, remember even though you probably pay for the phone, it she's responsible enough to have it, the law will state that she is 'of sound state of mind'.
Oh and yes, there is software out there, which will back sms messages. You will need her consent to access it, and keep it, otherwise you'll be breaking the law.
There is something in Commonwealth law about using a telecommunications device to menace or harrass.