I turned 40 this year, so my GP said I should go for a mammogram. I did, and a week later they called me back for another one, on a different machine. I asked the receptionist if I should be concerned and she said not at this time, but I can't help but to be concerned.

I was told in my 20's that I had dense breast tissue, so I'm hoping it's just that.

In all your wise experience, how common is this and how much should I be worried?