go see your gp to see if she has a problem with one of her neck muscles, it may be tight (torticollis) if so she will need referral to paediatric physio.
i have posted some advice on helen021's post about flat heads you may find helpful
also should have put on there that giving them encouragement to turn their head in the opposite directio to the way they favour is good. so talk to them/show them toys/have music/tv on that side so they are more likely to turn their heads