You can have a flat bump!
... Causes and Types of Raised Skin Bumps
There are many different conditions that can cause raised bumps to appear on your skin. The most common causes of bumps are harmless and do not require medical treatment unless you have discomfort:
Cysts are growths that develop under the skin. They feel like a small ball and can be moved around slightly.
Angiomas are blood vessels that collect together and create a raised, reddish spot on or bump under the skin.
Seborrheic keratosis are round spots on the skin that appear most often in middle age. They may be skin-colored, brown, or black.
Moles are ***flat or raised bumps*** on the skin that are usually benign and range from skin-colored to dark brown.
Acne causes skin bumps that can range from very small to bigger and painful. The bumps are usually accompanied by redness and swelling.
http://www.healthline.com/health/raised-skin-bump