You can't withdraw convertable potcoin balance, that's merely a transparency feature. Our security procedures do not allow for instant manual withdraws -they typically take 20 minutes to one hour- this is common with most pools. The onus is on you to ensure your balance is regularly withdrawn if you want to jump on speedy trading. If you were directly mining potcoin, and have a legitimate user issue, please do not hesitate to contact us via our support system.
Edit: in fact, that you are seeing a negative balance likely indicates the trade team was right on top of this pump.