When masturbating once I've hit the orgasm my vagina becomes very sensitive and I can't continue what's happening how can I stop this?

's normal. After the build-up and swelling and release of orgasm, the whole area of the clitoris, clitoral hood, labia, and outer 1/3 of vagina (these areas are inter-related) become quite sensative for a short period of time until the heightened nerve and blood flow response return to baseline. It is, qand you are, normal! enjoy your orgasms, but you have to wait a bit in-between...
Time. This is a normal response due to increased pelvic bloodflow. Usually resolves in 5-15 minutes.