I don't know the cause of the bump in your butt. That is the perfect area for a bacterial infection. Perhaps there was a minute break in the skin from the constant movement and contact with the saddle. I've had a similar bump but closer to the center line that eventually went away with daily cleaning and antibiotic cream. These days i make an effort to maintain the area very clean and do a scrub before a ride and a shower after. I wash bike shorts the instant I return home from a ride as bike shorts are a perfect environment for bacteria. Good luck.