Losing LP While In Series

Hey all,

I was recently in a promotion series for Silver 5. I lost my first game and I dropped down about 15-20 LP (I forget the exact amount). I thought this was strange since I never thought you could lose LP in a series. Then I played another game, didn't win, and lost some more LP. Another game lost, more lp lost. After it said series lost, I went back to check my LP and it was 40 something (forgot again ...). So, is this intentional or is this a bug?