Beware and be careful. It looked for a while that I would have to give up cycling altogether. I noticed I was numb after roller sessions on my fixed during winter 2012/13.
I couldn't be bothered to master rollers so fitted a device that holds the front forks so the whole thing acts as a sort of turbo. I fitted a big gear and went for it all winter.
It was early summer that I noticed pain in the (un)happy sacs. It wasn't a sharp pain, more of an ache that just wouldn't go away. I waited two weeks and then went to the doctors.
Thus began a time where more men cupped my nadgers than a gay porn star.
Doctors, consultants and scanning expert; they all had a go as it were.
Eventually, I was told everything (more or less) was normal. I had what was described as varicose veins in the testicals. They might get worse in the future but, then again may not. I decided on a self imposed embargo on riding during what was a glorious 2013 summer and didn't ride again until Spring 2014.
I have kept rides shorter than of old and have avoided the fixed totally.
I believe it to be a combination of the following:
1. The stem was too long on the fixed (155mm).
2. Position not having to change on the rollers.
3. Never standing up on the pedals as you would on the road.
I use the same type of saddle on all my bikes (Turbomatic II, III or IV) and don't believe that to be the problem. You cannot believe how much I wish I hadn't ignored it. My personal advice would be if on the turbo get off and walk around to get the blood moving again. Once the numb/tingling sensation has gone, carry on.
Look at your position rather than the saddle assuming it doesn't bother you normally and you use the same type on all your bikes.
Sorry for the rather long post!