This is most likely due to a settled bet which has been credited to your account that has been resettled as lost. In these cases the incorrectly settled winnings are withdrawn from the account leaving your balance negative if you have already made a withdrawal or played the winning into another game. Section A.5.6.2 of our terms and conditions supports this action.
The only way your balance is going to be positive again is if you win or make a deposit to correct the balance again.