I have had this happen to and am not sure what the cause is. I dont think its anything to worry about. It only happens when you are not erect right? I noticed it seems to happen more often hours after sex.
I assume you're talking referring to when your penis is flaccid. The penis changes its shape and size depending on the temperature, just like your ball sac does. I don't know why the penis would need to do that, but it does. Well it does on me at least.