What to do if i have a pimple- like bump on my breast. what could it be?

Several options: Depends on the location and appearance, wether tender or not, and how long present. Could simple be a pimple or incrown hair. Could be a small sebaceous cyst, or small mole. The nipple has glands under the skin that may become blocked. Most things not worrisome. If it persists, is tender, or gets larger I wouldn't hesitate to have it checked.