Penis Shrinkage and jogging

When I work out or play sports (or get nervous), my penis retracts almost completely inside.  Any other time, I am happy with both my flaccid and erect penis size.

This has only been happening in the last 3-4 years.  I've read some other posts from guys claiming the same symptoms.  What I'm looking for is a name for this, what is the underlying cause, what I can do to combat it, and will it ever go away.  

I am slightly overweight (I was much less when this started), 30 years old, I was a smoker for 8 years when this started but quit not too long after this started, I am in pretty good shape aside of being overweight (I work out about 5-6 days a week), don't do drugs (prescription, over-the-counter, illegal), I drink but I'm not an alcoholic, and have never had any physical trauma to the area.

Some sites have suggested "Penis Panic" as the cause--A syndrome where men from islands in the Indonesian area believe their penis is shrinking.  I'm not imagining this.  It is real and is very uncomfortable.  

It's hard for me to jog through the neighbor anymore because I have to constantly "adjust" myself and my 'hood is pretty lively.  And obviously, this is rules out football or basketball at the park.  Most importantly, I'm a musician so I'm constantly on stage in front of groups of people.  I never had these problems before a few years ago.  I don't know if the shrinkage occurs on its own or, in the case of being on stage, has the nervousness become a mental trigger in that situation.