1.
Overlimit: The credit limit of the bank is exceeded when swiping the card, because some credit cards of Ping An Bank support the over-limit limit.
2.
The sum of interest+principal generated exceeds the bank's credit line: you may not have paid it back last month, and then it has been maxed out this month, so the sum of your principal+interest exceeds the bank's credit line, which will also generate a negative number.