She is a potential squirter.
Her G-spot has engorged and is ready to ejaculate when she feels that sensation.
Make sure she urinates before coitus. Once she experiences that feeling, tell her to let go and push.
She will either gush all over you or actually ejaculate.
The more she does this, the more pleasurable it becomes for her.
The sharp pain could be you hitting her cervix. Try to stroke her so you slide past it into the deep spot. Once there just nudge fuck her a bit.
Her eyes should roll back in her head and she should start speaking gibberish.
Then tell her to come for you.
Cheers.