looking for a story

i read a story ages ago, and i thought i'd bought it online, but can't find it on my list on lulu or amazon.

basically a young man sleeps with girlfriend, thus unknowingly causing her to lose her ability to do magic, her mother seeks revenge turning him into a woman, and he unexpectedly ends up with great power.

i hope this rings a bell for someone, the story came to mind several days ago and i can't seem to track it down.

regards

Amanda