Silver V demotion?

Hey lads! I'm so sorry I'm asking this here, I know the info has to be somewhere but I keep looking and just can't find it. I just got demoted from S4 to S5 after a loss at 37LP. I was earning about 25LP per win so my rating must have been pretty good. HOW does this happen? How on earth can I get a demotion when I was at 37LP? The only different thing from usual is that I hadn't played for 3 days or so, but that can't be it right? My account:
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