I am always telling people that ANY change in your breast needs to be looked at promptly!
My situation was that I noticed an indentation (lemon no 2, top left in that pic) at the bottom of my breast. Could only see it in the mirror because of the position of it. Luckily I did go straight to my GP. She could not feel a lump, I thought I could. Nevertheless she sent me for a mammogram and ultrasound. She was shocked when it turned out to be cancer!
The associated lump could barely be felt and yet it had spread to my lymph nodes extensively. I will always wonder how long the dent was there before I just happened to notice it in the mirror that day! At only 47 and with no family history or known risk factors I had never even had a screening mammogram. The tumor location was unlucky and despite having a mastectomy, clear margins were only obtained by the surgeon taking a 'shave' from my chest muscle. Thank-goodness the muscle was clear of cancer.
I knew how to feel my breast for lumps but did not regularly examine them in front of the mirror! :(