How is your position on the bike. I noticed a big improvement (no more pain) when I raised the bars pretty significantly. I thought that the bars needed to be below my seat, but that's not good on a spin bike. Raising the bars puts less pressure on the parts because I'm not leaning forward so much. It also feels better on my back. Same good workout, without the hurt.

Also make sure your seat isn't too high that it causes you to rock side to side. Speaking of rocking, when you spin fast, do you bounce? Alot of people do and it's not good. Good form is more important than cadence of 120+. Keep good form under control. Hopefully this will help.

Oh, and I do wear cycling shorts.