I don't know what it is, but it sounds exactly like the issue that's been bothering me for a long time.
For me, it would come on at about an hour's saddle time, sometimes it fades away again and that's it for the rest of the ride.
I changed saddle recently to one that's much flatter with a deep curve running up to the nose section, thinking I'm pinching something between the rounded edge of the saddle and the sitbone. It seemed to help some, but not totally. I'll know more when spring comes and I can ride again.