I had a cluster of calcifications and I was told it was pre-cancer. I like you thought the worst, but I ended up having a biopsy and it turned out to be benign tissue. I suggest having the biopsy so you have some comfort level. All of this started when I went for a routine mammogram that spotted the calcifications. I ended up going to a Breast Surgeon who scared the daylights out of me but they told me that with calcifications unless you have a biopsy, they don't know know whether it is benign or malignant. I know exactly how you feel because I was exactly in your place! I learned it is not worth worrying until there is something to worry about. You can do all the research you want, but until you have the biopsy, you won't know definitively what is going on. Keep your chin up!
Are you sure that the nurse said that "it was the start of Cancer?"
Ordinarily, this sort of sensitive information is not imparted by anyone other than your doctor... in person... in a consultation room... and, then, only after reviewing pathology results following a biopsy.
Perhaps, you were told that a possibility of Cancer was a concern and further testing is needed.
In any event, you have yet to have an evaluation (probably, more diagnostic imagery) of the cluster; and, then a determination will be made as to how to approach your condition. So, you don't want to get ahead of yourself and jump to conclusions.
Now, since you have a known history of fibrocystic breasts (a density that makes viewing mammograms difficult) and you've gone through the stereotactic biopsy procedure for the 2001 calcifications, yeah... it's quite possible that what's appearing in your left breast is the same as what occurred in your right breast back in 2001. But, no one can assume that it's the same... only more diagnostic tests and a biopsy report will confirm that.
Take care, good luck with your evaluations, and take your doctor's advice... don't "worry."