Shortin' On A Friday

Ok so this morning the play was shorting 99 Cents Only Stores NDN. They had a huge run in the last month, did report good sales but they got up to over 100 on the price to earnings ratio. That's a little too much I think. So I shorted it a few shares. Was this a "buy on the rumor sell on the news" run up? I don't know. It did open up today then took a nose dive right after the open. Note to self and others: I'm probably wrong!