What Is The Treatment For Pimples On The Buttock?

Hello,
I have three spots on my buttocks. Each started out similar to a pimple, but wasn't raised as much. Within a few days they turned into flat dark blue/purple/dark red spots which seemingly go fairly deep into the skin. They are about a half inch round. Two of them have been there for a month or so now and the third just appeared. They did not hurt much when they first appeared.
I am wondering what they are and why they appeared? They will not go away! I do ride horses, so I was wondering if they were caused from riding?
Please help! I cannot find information about it anywhere...