Dermatofibromas don't usually grow that rapidly, although it may have been there longer and you didn't notice. It may be a cyst or a piece of fibrous tissue. Blanching on pressure just means there are blood vessels in the skin.
My advice is to leave it alone--no poking or prodding--and let a doctor take a look in 4 weeks if it's still there. Your description does not sound sinister to me, but I guess there's no way to tell without looking.
Take care.
Dr. Rockoff
I have been wearing a lot of waterproof suncreen and I was wondering if this could be a blocked pore. Can they get to 4mm in size with no discomfort?