After being nice to a fairy creature a Granny is granted the gift of being able to turn into a young magical girl. She seems happy about it until she discovers she now has to fight demons running amok. I originally noticed it on ExHentai than traced it to this facebook of a Chinese author.
https://www.facebook.com/pg/animesto...89984955012948
I've seen a few other manga like this one
http://www.mediafire.com/file/sdggg3..._Girl.zip/file