Of course you are frightened. We've all been in your shoes so we know exactly how you feel. My cancer was also diagnosed in a routine mammogram. Don't worry about size. 17 mm is 1.7 centimeters, which is how most tumors are measured. That's not very big... just a little over 1/2 inch. If it does turn out to be cancer, sounds like they see it early. What kind of doc called you back? Since you don't usually make appointments with a radiologist, I'm think this was some other kind of doctor. If it was a radiologist it would be more significant.
There are any number of reasons for a call back. Sometimes it is a matter of technique. One mammo technician does it a little different than another. (That happened to me one time.) It could be because you have dense breasts and hard to diagnose. It was described to me as the difference between a bowl of plain jello and a bowl of jello with fruit cocktail in it. You can't see all the way to the bottom plus you can't see what is under a small piece of fruit - a cherry? A grape? If you have dense breasts AND less compression, for instance, than you did last year, the radiologist can't make a good comparison so they bring you back in.
It's likely that you have calcifications...pretty normal at your age. They show up on a mammogram as bright white scattered round "pearls" and are a common part of aging. The radiologist looks for odd shapes or tubular shapes like rice. He/she also looks for a few of these calcifications in the same area.
Your next mammogram is likely to be a magnification of a smaller area. If it still isn't clear or still looks suspicious, the doc will order a stereotactic breast biopsy. It's completely painless. You feel a small sting of lidocaine, but nothing else. They will numb your whole breast, compress it, and direct the biopsy to exactly the area in question. Then they will put in a tiny titanium clip to mark the spot where the biopsy was done and you will move into another room for a regular mammogram to make sure they got the right spot.
It takes a few days or a week for the pathologist to look at the samples ( I had 12 taken ) If it is cancer, you will be given your exact diagnosis and be sent for an MRI with contrast to look for anything else that might have been missed in either breast. From there you see a surgeon and the ball really starts to roll.
You have a way to go so there is no point making yourself sick over it when you'll likely have to wait for what seems like forever. Hopefully the magnification mammogram will clear up the question and you'll go back to your life.