Are you sure you didn't accidentally click your BTC balance when sliding in a buy order? I know it sounds dumb and an elementary mistake, but there have been plenty of people (including me, once) who've done it. Otherwise, Poloniex is a pretty well-developed site and I doubt there would be glitches like that in their system.
In the left panel where it says BUY i have entered amount of X to buy (10000) and it used all my BTC to buy that X coin. Where is the option to click BTC balance and is it better to use the STOP-LIMIT panel then?