The term for the tip of the penis is "glsns", and sometimes during a guy's lifetime, they do start to "lean" backwards. Usually it' doesn't lean backward too far over vertical but if yours is doing so, I'd see a doctor.
Are you saying your urethra is slowly shifting away from the opening?
That sounds like something you should see your doctor for. Masturbation shouldn't cause this.
It's kind of hard to understand what you are describing...sounds like it could be perfectly normal, though. Have you tried looking up "medical image of normal penis" or something like that on Google? Like all body parts, there will be variations all considered "normal."