I just won a ranked game where i duoed with a friend. He is plat so obviously i had to play up with people in his rank. After we won the game we both lost LP. Can anyone explain what the heck happened?

It's probably the graphical bug that's been happening for a while, it should update itself before too long.

A lot of people have been complaining of this issue, but then say that it fixed itself.
Here, and Here.

you sure because i legitimately went from 42 down to 26

I just lost a game. Got 24 points.

http://i.imgur.com/Oepn9gn.png