I doubt this is cancer. Remember the breasts are glands and the veins are close to the surface, you may have just bruised it in your sleep without knowing, or maybe the bra you are wearing is causing the problem. If you are really worried, you can have tests done, but I would not be too concerned. Veins close to the surface are more prone to bruising
I Have Noticed A Bruise-like Mark On My Breast That I Can't Understand Where It Came From, Had One In The Exact Place A Few Months Ago Too. Is This A Symptom Of Breast Cancer?
Common symptoms of breast cancer are, nipple retraction, skin colour change, ulcer, lump, nipple discharge, breast fixation and skin tethering are common symptoms