If you are riding more lately than you normally do, I would guess it is probably that. Were you going over particularly rough roads (i.e. ones that would cause constant vibration)? If you aren't used to riding 75 miles in a short period of time (and particularly if your saddle isn't fully broken in), you might experience some pain. I can tell you that when I end up riding more than I plan to (and thus don't go into it properly prepared), I do experience numbness to my man parts.

Generally, you can do things to prevent pain and numbness down there but sometimes you end up biking more than you plan to and them's the breaks. It's happened to me many times.