R.Y.
Take a breath and don't freak out. Just because there was a report to dcf doesn't mean they will find a problem. They legally have to investigate everything even though some reports don't have any substance to them. I used to work for the similar agency in my state. Your best bet is to cooperate, tell them you talked to the pediatrician, etc. They can't tell you who made the report but ask what the referral was specifically about (they can tell you that part). They will probably talk to the pediatrician and the daycare as well.
See if they can give you a referral to get an evaluation of some kind if it is about the head banging. Head banging happens often with some medical or behavior disorders but it can also happen by itself and not be part of any serious disorder as well. A pediatrician, developmental pediatrician, or psychiatrist would be a good place to to start if you want an evaluation.