Wrong saddle. Or-
Low hours on the bike- way back I got sick and was off the bike for almost 6 months. When I resumed riding, my previously comfortable saddle was anything but comfy.
30 minutes was all I could tolerate.
In general, when I see a saddle so set up and I meet up at a stoplight with a rider having a saddle so mis-set, I ask about their lower back. They all complain of discomfort and then ask "how'd you know?"
As they are riding they are constantly attempting to overcome the slide they are on.