Me too Leesa! My history sounds exactly like your's.

I had a lumpectomy 5 years ago where the surgeon removed a chunk of tissue and left a caved in area under one breast, right where my bra sits. It took around 3 years before I could wear a proper bra again (I had to wear sports bras that looked like crop tops), and even now I still can't wear underwired bras without them hurting the scar line.

I too had a nasty sharp pain like you described. I went to the doctor about it and was told there may have been a trapped nerve in there somewhere but I really doubt that's what caused it as the pain gradually eased. I think it took about 3 or 4 years all up, and it no longer bothers me.

So I can't explain why it happens, but I guess it must be a costochondritis of some kind, caused by the surgery. Anyway I did want you to know that you're not alone and I hope your pain eases soon.