You most likely had a chargeback or refund. As per our terms chargebacks and refunds are deducted from your account. You can check this by logging in to your account and click the Stats button and then click the Detailed stats link on the stats page.
On the next page you select the program (Pay Per Signup or Revshare) and press the OK button. You will then presented with the detailed stats page which will show sales (S), rebills (R) and credits (C) = refunds and chargebacks. Obviously we can’t pay for sales that got refunded or charged back, so those amounts are deducted form your balance.