Being a man the closest I've been to that is that my wife has experienced similar. She has a small lump in a similar location, it's been there since we got married (before actually) 31 years ago. It's not cancer and has never changed. Her sister has lumpy breasts (apparently, not that I've experienced personally) they've talked about it to our 3 daughters.

I was diagnosed with cancer 6 months ago, I had a period of time where I was waiting for a diagnosis of my issues and even although cancer hadn't been considered at that time I still worried about it, so I understand that. The thing is, I think to some degree most of us will worry about the "C" word from time to time usually without any need. Your GP is probably right that it's nothing to worry about and you'll soon find out. Easy to say but don't spend your time worrying, it's more difficult just now but just enjoy your world and once you've got the all clear continue enjoying your world.

all the best 

Ffoggy