Probably a bite or a pimple. However, if it does last longer than a week or so, and particularly if that breast shows any sign of inflammation (hot, red, enlarged or swollen) or you notice changes to skin texture or nipple position, then do see your GP urgently because red marks and rashes can OCCASIONALLY indicate a kind of breast cancer. This would be rare though so don't worry - it's just if it persists or you get the other signs mentioned above.