It may be that it's "one of those things" and will sort itself without issues. but if in two weeks or so it hasn't changed, or has grown, or any other worrying signs like nipple discharge, pain, puckering of skin, etc, or lumpiness under the arm, repeat your request until they listen. I had a breast lump after stopping breastfeeding my daughter, but they wouldn't biopsy it as I had an implant for a chest deformity, but that's now years ago. Last autumn I found a lump in that breast, watched and waited, kept checking, then it turned sore if I touched the area, couldn't get through on phone to GP so stood at the reception desk, without an appointment, and said loudly that I needed to see someone urgently due to the lump, got seen, and was immediately referred, seen two weeks later, got the result of biopsy within 10 days and planned surgery done 3/12/19  I hope yours turns out to be something not to worry about, but for now, you've come to the right place to ask and get informed support, the nurses on here are brilliant.  WIshing you a peaceful Christmas and best of luck. 

Call them over and over again! Don't let them fob you off, I wouldn't.Keep calling and say you will sue if it is cancer.My husband was sent away 4 times with a cough that was worsening over a course of a year.He was told the cough was normal! He had stage 3 Hodgkin's Lymphoma and we are at the final stage of the medical negligence case.

Don't let them ignore you, it's not on!

Good luck!

Oh my goodness how scary I hope he is okay? I'm sorry to hear that. So awful that he had to keep going back for them to listen. 
I have decided just to go private instead of wasting what could be valuable time!