Whoa man.
I was a huge fan of both of those fics (MCR not Big Time Rush- I don't know who that is actually.)
It's pretty low to steal someone's stuff but did the other author actually say it was stolen? Maybe it's the same person, or maybe she got permission to switch it in to an MCR fic because sometimes fics written for other bands can be cool to write, if you have permission.
There are a lot of variables in this that have yet to be explained.
Author's response
If it was a misunderstanding/ she had permission, she wouldn't have deleted Well you can hide alot about yourself after she saw my post, and she would have either posted an explanation or wrote me a review telling me I'm a skullfuck and that she has permission...