Demoted from one loss when I had 50 LP

It was only from silver four to five, so not too big of a deal. However I was at 50 LP and would gain or lose about 20 LP per game. So I had 50 LP going into a veigar game. Our shaco dc'd at level 4 and came back when we were taking their first inhib turret (winning), and the enemy caught us out at baron to take the win. Very unfortunate, however not the issue. The sudden drop of a division made me not only angry, but confused. Is there a bug in the ranking system?