I think it's just poor circulation to your groin. I've always had the same problem since I was a kid. It's no big deal since they will regulate themselves (by moving up and down=towards or away from your body) for the correct temperature control.
So I've worn breifs all my life, fathered two children, so that kills that subject.
So the next thing is does this reallt affect your life Mr coolballs, if not, dont worry about it and just get on with your life.
Good Luck
Wearing boxers instead of briefs increases sperm counts BECAUSE it allows them to move away from you when they get too warm and toward you when the surroundings are too cold for them. The reason the testicles are not deep inside your abdomen like ovaries are located is that sperm are healthiest when they are cooler than 98.9 degrees. So if you hold them and warm them up, you may as well keep wearing tight briefs that hold them against your warm body.