I don't think thats natural...
it is, it happens to me sometimes too and it hurts like a bitch. worst thing is that i have no idea what to do when it happens, because when it does, i'm usually at school or something and i can't just shout "AH, ONE OF MY TESTICLES HAS CURLED UP, WHAT TO DO, EVERYONE???", nor can i let them know i'm in pain because they will ask about it. of course i could just lie but it'll feel weird.