What Causes Curvature In The Penis?

I, Bharani, am a 22 year old male belonging to India. Quite often my flaccid penis gets a dark wrinkled tapering mid-way of my penile shaft, giving it a hour-glass shape. As soon as my penis is fully erect, the mid-way tapering disappears, but my fully erect penis, which might of a length of around 5.5 inches, curves downwards a bit, resembling an inverted thumb. I ejaculate normally (about 4 ml), and also experience orgasm normally, but I don't understand this problem. Please help me.