Hi C.,
My daughter did this too when she was that age. I asked the Dr. about it at the time and she said not to worry. Now my daughter is almost 4 and has been diagnosed with an autism disorder called Asperger's Syndrome. Turns out it's really common in Autism disorders, and it's a red flag to look out for. She quit the head banging when she was about 20 months old when we started taking her to a chiropractor, but now in the last couple of months she has started smacking herself in the head/face and clawing at her arms when she is frustrated. SO, I am not trying to alarm you, however, I would do some research on autism (just google autism symptoms) and see if your child has any of the other symptoms. If so, get her evaluated as soon as possible. That being said, she might be a bit young to really be able to tell very well if she has symptoms, but it might be a good idea to just keep them in mind to watch out for as she gets older.
It could also be a symptom of something called Sensory Integration Disorder. She could be hittting her head because it provides sensory input for her. Does she like to be tickled a lot, spinning around, rocking, flapping her hands, etc.? (Most all kids this age like this to some degree, I am talking about a more than normal desire for this)Does she seem to want to play with water, sand, playdough a lot? (Again most kids do at this age, I am talking about an unusual interest in these things.) It could be that she is under-sensitive to things that most of us process with no problem, and she is craving more sensory experiences throughout the day. Sensory Integration Disorder is part of autism disorders, but it can exist independently of autism and just be that itself.
Or it could be nothing to worry about at all. I would just be open to exploring the possibilities and seeing if there is anything there. If I had it to do over again, I would have checked out things further instead of thinking everything was okay only to find out the hard way that my child missed out on the early intervention she could have had to help her. I wish someone would have pointed out the red flags to me when I was expressing concern. I sensed something was wrong with my child from the time she was about 6 months old, but everyone kept telling me she was fine and I was worried about nothing. She would quit her odd behavior, and catch up in the areas she was behind in. "Every child is different" "Your daughter is ahead in other areas" "She's just going through a phase"
My point in telling you this is that you are her mother. You have intuition regarding your child that no one else has. Trust your instincts. I wish I had listened more closely to mine. If you are worried, check it out further, If there is another issue, you will find out early. If not, you can put your concerns to rest and be done with it. Either way, it doesn't hurt to do some further research.
Good Luck!
-L.