Hi Liberty,
I too had abnormally dense breast tissue (at age 24!). My doctor at the time was very concerned and sent me for a bunch of further testing. They found nothing to be concerned about. I've had two other doctors since then who have both agreed that it is just normal for me, and that when I start regular mammograms they may have a hard time reading them. So, chances are they are just being extra-careful (which is a good thing). I will still pray for you, because I know what the waiting is like. Sorry you're going through this.