I had a small cancerous lump behind my left nipple.  Lymph nodes not affected .  (My left breast has always been bigger than the right one; I think this is quite common and this is why most breast cancers are in the left breast). I hadn't felt a lump but it came to light when I had a routine mammogram.  I was told the prognosis was very good.  I had 12 rounds of chemo, with immunotherapy, then 4 rounds of ec chemo, again w immunotherapy.  Then a lumpectomy ( the scar is barely visible because it is on the lower side of the areola).  Then a week's radiotherapy.  Then a few more rounds of immunotherapy.

Of course "your" lump may be nothing to do with cancer eg a cyst and I gather the majority of cases bear witness to this.  

I do hope all goes well with you 

Yes I had this and it was a benign cyst. They left it alone but it became infected but never turned into more than that thankfully. Good luck to you x