What happenes if you go too far up your vagina when mastubating?

As long as you are masturbating gently, there is little risk of going "to deep", but if you are careless or if you are too forceful then you can hurt yourself.

If you go too deeply you can hit your cervix, which would cause a sensation in your abdomen that would feel like getting punched in the stomach. It is also possible to break through the thin wall of the vagina and into the abdominal cavity or even into the rectum. This could lead to a dangerous infection that could be life threatening.