My observations.  

Last fall where I put in, there is a shallow sandy rocky bottom which slopes to the shore.  There were no beds on that bottom the last day I fished.  Late October or early November.

The first time out this spring, mid March, there were wall to wall depressions in this bottom.  Enlarge the surface of a golf ball, and you get an impression of what this looked like.

The depressions were all the same size, maybe a bit over a foot in diameter.  They were obviously made by critters.  They were not new.  The depressions were filled with decaying leaves from last fall.  My guess is they filled with leaves after something had finished using those beds.

Now the bass are on their beds.  Most that I have observed have a single bass standing guard.  But, in places the bluegills and other sunfish are thick along the shore in many of the same areas.

Also, where I put in, I have recently noticed small fish, maybe two inches in length max.  They have the shape of a bass, but are too small for me to make out any characteristic markings.

Any thoughts on what was happening.

Doesn't seem to make sense that any fish would spawn with winter only a couple of months away.  But it appears that is what happened.

Bullfrogs digging in and hibernating, or sun turtles?  Thought they wintered over buried in mud.

It sure sounds like a bluegill spawning area but I'm not aware of them spawning that late in the year.