What Causes Painful, Green Bump On Back?

I have a green bump on my back. It hurts when I touch it and it's very hard. I don't think it's a pimple. It's been there for months. I researched online and the closest answer I got to was a pus or swollen lymph. I looked at the pictures but it's not that severe. It's been bothering me because it hurts when I put my back against a wall.