What Causes Bump On Penis?

Hello , i am 14 years old and while i was taking a shower i found that every time i was trying to clean it , it hurted so i checked and there was a little ball which i consider a pimple. a pimple doesint hurt as much as this little thing hurt. even if i pass a finger thru it , it starts hurting. yesturday it was my first time seeing it and today is not as small as it was yesturday. so it got bigger - im really scared . i was thinking it was some HIV but i hope is not. so what could it be ?