Cysts can change over the course a woman's cycle, so if the doctor suspects a cyst, that might be a reason to want to see you mid-cycle.  Someone at the doctor's office should have reassured you that call backs for follow up mammograms are extremely common.  It just means that the radiologist wants a better look at something on the mammogram.  It is good to have a doctor who is super cautious.  Even if it turns out that this is a solid nodule that needs a biopsy, the vast majority of biopsied lumps are not cancer.  

I do understand your anxiety because I have been there myself.  I would suggest you call your doctor today and explain that you are having panic attacks.  Your doctor can probably call in a prescription for an anti-anxiety medication to help you over the next few days.  Or try going outside for some exercise or doing some fun activity that will take your mind off your worry.

I'm so glad you called and got the appointment scheduled for an earlier time.

 Mass seems to be a more general term than nodule.  I have heard doctors use it to describe both lumps and thickened areas.  A nodule is a lump.  The vast majority of breast lumps are not cancerous.  Many are harmless fluid-filled cysts, especially in women in their 30's and 40's.  An ultrasound will be able to determine if a lump is a cyst most of the time.  Even when the imaging tests are not sufficient to rule out cancer, most biopsied lumps are not cancer.  For every 100 women who need to have a nodule biopsied, more than 80 will not have cancer.  

Of course, I can't promise that your lump is benign; some lumps are cancer.  If yours is, it is still very small and there would be every reason to think that you would be able to have breast-conserving surgery and that you will live to be a very old lady.  In your forty-five years, you have developed all kinds of resources for coping with crises.  If you get the worst possible health news, you will reach down into your inner resources and out to your community of support, and you will get through this.

What you need to do right now is get through the next two days.  Make yourself eat simple, nourishing food.  Avoid caffeine or other foods that might make you more jittery.  If your doctor prescribed some anti-anxiety medicine, take it as directed.  Try to stay busy.  When "what if" questions pop into your mind, write them down to ask the doctor and then try to forget them.  Once you get the report from the next set of tests, you will have enough information to know whether it is time to panic yet.  I hope you get very good news and can put this scare behind you.