Hi MGoodall,
If I were you I'd be back for a second opinion. The fact that it's in your head and driving you insane means that you really can't let it lie. The chances of it being anything sinister at your age are extremely slim, but it's not impossible, so for your own peace of mind, please go back and discuss it with your GP. Does he/she know that you have a family history of BC and that your obviously very concerned? You'll make yourself ill with worry if you don't address this issue, so please go back to see your GP. Ann x