I have a bruise like blotch that seems to be under my skin, it appeared over night on the top of my breast, what are the symptoms of skin cancer etc?

Probably not cancer: Skin cancer is rare in someone 23 years old. I wouldn't worry about it. The symptoms of skin cancer are change in or development of a mole or skin nodule that is raised or flat, asymmetric, discolored, and that grows in size. Your symptoms should go away on their own within a few days.

Probably OK, but...: If this fades quickly, it's likely a bruise or broken blood vessel. But there is rare type of breast cancer called inflammatory breast cancer that can start with bruising like this. If you have pain, pressure, a heavy feeling in your breast, or if the discoloration doesn't fade in a day or two, see a doctor immediately. It would be very rare to get breast cancer at age 23, but it happens.

Identify cause first: Post traumatic bruising is the most likely cause but best to see your pcp or breast surgeon so that a full history and examination can be undertaken.