Has anyone ever had an abnormal mammogram? When you had it redone, what did it show? I am wondering if it happens sometimes that the second one reveals no problems even if the first one is suspicious?

My mammo earlier this week showed "changes" from my last mammogram a year and a half ago and they want me to come back for a more thorough diagnostic mammogram and an ultrasound. I'm scared this could mean cancer. They did tell me that I needed the second mammogram on both sides, so I thought maybe that was a good sign, because if I had a tumor, wouldn't it just be on one side?