Because of gravity.

You better get yourself off to the doctor immediately. Melanomas pop up suddenly like this (I have never heard of one on a penis before but it would happen) and if you don't get it looked at immediately then you're history. It's got nothing to do with sexual activity.

I'm going to assume you mean "vagina". Va⋅gi⋅na  /vəˈdʒaɪnə/ –noun, plural -nas, -nae  /-ni/ 1. Anatomy, Zoology. A. The passage leading from the uterus to the vulva in certain female mammals. B. A sheathlike part or organ. 2. Botany. The sheath formed by the basal part of certain leaves where they embrace the stem. You might … Read more

Goodness I don't know. I've never heard of this before. It might be best to talk to your doctor about this one. It also might be a smart idea to post it in women's health section. You might have better luck getting and answer there.