Mysterious body parts

I was sick for three days in a row this week. During that time, or at least I think it was, I noticed a few things on my lower back. I didn't know what to call them, and I still don't, but I guess they might be considered bulges. But they weren't a goiter or tumor or anything, because I've seen them before. They are the two bumps that are directly above the buttocks (did that sound formal/doctoral or what?). I didn't know what they were, so I felt them. They felt kind of like muscle, I thought. This presented another question: How do I flex them? It must be true that all skeletal muscles can be flexed, but I couldn't figure out how to do it. After perhaps five minutes of bending my back and kicking my legs, I figured it out. They are above the butt because they are (kinda sorta) part of the butt.

This changed my view of this muscle, seeing that we guys are showing our butt to everyone at the swimming pool or beach. It wasn't a big problem to me; I didn't quite care, but it is still interesting. I wondered what other muscles could be extending further than I had thought. But then I threw up, and the moment was gone.