Is her evening meal higher fat and/or higher protein than previously? Either of those can cause a slow rise over up to 8 hours, and they each double the effect, so for example, 30g protein will cause a long, slow rise, and add 30g fat, you will get an additional rise of approximately the same amount.
It is also true that teenagers' insulin requirements can be quite high so it may just be a marker of puberty - or a phase in her menstrual cycle.
If her daytime readings are not just flat but in the right zone (not too high), while her nighttime ones are rising, she might need to change to two shots a day of Levemir (I am sure, having been a teenage diabetic, that won't be popular) or ask to move onto a pump.
But look at what she's eating for dinner first.
(And look under the bed for the sweetie wrappers (kind of a joke, but I really was a teenage diabetic, secret runs to the biscuit tin and all that). Going from 6pm till morning without eating at all is a long time for anyone, let alone a teenager. Might even be best to give her a bedtime snack and another bolus.)