I had a lump in my left breast which went of its own accord and didn't show anything bad on a scan I had. That breast has got noticeably bigger than the other one but not enough for me to see a GP as most people have one bigger than the other.
However, for the last couple of days I have had a pain in my breast and it feels almost like a marble but there is no lump.
I am sure everyone would say go to the GP but ime they send you away to see if it is linked to your periods. Mine finished a day or so before the pain started.