Re: Decrease in size of my penis.
I had same problem like yours in my youth.
My cause was obesity. My penis decreased dramatically. According to my doctor, the shaft is not decreasing, but it was shrunk by the fat in pubic area (between penis and belly button). He told me to arrange my meal time properly and have a diet. Exercise, so I took swimming.
Amost 2 years, I returned to normal again. The penis back to slighty normal like before the obesity.
Anyway penis length is vary in size, each session of the erection will never be the same.
There are numerous other factors that can cause penis decreased in size, such as -- age factor, distress level, tiredness, disease, etc.
Hope this helps.
My suggestion: drink fresh water before sleep, and wake up before brushing teeth. Swimming is the best exercise.