I'll mention a couple of things that could also be happening. As bounceswoosh mentioned your core could be weaker and your fitness level not the same as when you left it last year. If your core is weaker you aren't able to support yourself as well and you could be leaning on your arms. Instinctively you want to push away from that so you are scooching back. Simarlarly, your core can't support well and after time you start to slouch onto the sensitive parts instead of maintaining posture on the sit bones. If also your "aerobic capacity" is not as good this time of year, you could be getting tired sooner and maybe coasting more or not pedaling with as much power. The pressure on the pedals with constant cadence would help offset slouching on girl parts. You may need to find a new comfortable position such as a slight saddle or stem adjustment.

But then, it could be the saddle as well. I had one saddle that I could ride very well up until about 38 miles, then it hurt all the miles after that. (But that was sitbone hurt, not pressure elsewhere). I changed my saddle and I could ride much more than 38 miles pretty comfortably.