What Causes Erect Penis To Curve Down?

I am a 22 year old male belonging to India. I have got to know about you while surfing on the net for andrologist. Madam, this is to discuss a very intimate and crucial problem faced by me. The matter is regarding my penis. Quite often, when my flaccid penis starts getting erection, I notice a strange dark wrinkled tapering mid-way of my penile shaft. As soon as my penis is fully erect, the tapering disappears. Also, when fully erect, my penis, which might of a length of around 5.5 inches, curves downwards, resembling an inverted thumb.