I'm 15 yrs old and there is a red bump the size of a penny on the top left side of my butt near tailbone. Ive had it for about 8 months. It only hurts when I sit on it or lay on it or touch it. Its soft and its gotten bigger since the first pain I had. I believe I got it by sitting on my tailbone while on the computer. Any ideas of what it is or what I should do?