Waiting for ultrasound/biopsy

I hope this is the right place to post this. I have been reading for a long time (years!) but this is my first post.

I was diagnosed with IDC six years ago, when I was 39. I found the lump myself in a self-exam and it was quickly confirmed by u/s and biopsy. My tumor was Stage 1, 1.1 cm, ER/PR+, Her2-. I had a lumpectomy followed by four rounds of chemo (taxotere & cytoxan), radiation, and finally Tamoxifen. I am still taking Tamoxifen.

Earlier this year I was finally approved by my insurance for the BART genetic test, and I came back positive for two mutations (Chek2 and Bard1). Lucky me. Needless to say, if we had known this when I was first diagnosed, I would have had a mastectomy and not a lumpectomy.

However, I have been having alternating mammograms/ultrasounds and MRIs every six months all this time and everything has been looking fine. Until this last MRI at the end of November. The radiologist called me within a couple of days to say that there was an area that was enhancing and I needed to have an ultrasound. I asked specifically where it was and how big (1.4 cm, on the same side that had the lumpectomy, but in a different quadrant).

I was not able to schedule the ultrasound and probable biopsy until 12/23. It's such a long time to wait. In the meantime, I have been trying to locate this spot which sounds pretty big to me. I am small breasted and even smaller on that side where they already took a chunk out, so there's not a lot of room for anything to hide. In the quadrant which the radiologist described, I can feel a lump. It is firm and the right size, but it is also oval, smooth, has distinct edges, and moves around. It definitely does not feel one bit like my IDC, which was hard, bumpy and completely immoveable. From reading, it seems most like a fibroadenoma.

So I know I just have to wait it out until the next exam to know for sure. What I am wondering in the meantime, is with my history, am I just fooling myself by thinking this could be something benign like a fibroadenoma?

I am now 45. Most articles I have read indicate that fibroadenomas are found in much younger women. Am I too old? (I'm still premenopausal, if that makes any difference).

Could a fibroadenoma grow so quickly that it wouldn't have been seen at my last mammogram & ultrasound in May?

How specific is MRI at distinguishing a benign from malignant mass? The radiologist was obviously concerned to call me so quickly instead of just sending the report to my doctor to handle.

If anyone has any insight or experience with something like this, I would appreciate it. I have been through BC before and survived so far, but it is no fun to think of potentially going through it again. Thanks!