Just the other day when i was masturbating i realized that my penis' skin wasn't going all the way back. as i was doing it i saw that the skin completely covered the head and there was some extra skin. ive tried to pull the skin back while its erect, but when i do..it stops at the end of the head. and i feel that if i pull down harder it might rip....im getting kinda worried because when i have sex how will the skin go all the way back? is this normal?