Lost promotion series with one loss.

So, I won two ranked games in a row and get into a promotion series. Next day, I play one game and lose. Afterwards, I get a message pop up saying I lost my series and will have to work my way back up. I checked my match history, the last three games were two wins in a row followed by the recent loss. How is it possible that I've lost my promotion series? I'd like to ask for help, but I realize most likely it wont happen. At least make sure it doesn't do this again, yes?