From the outside looking in that seems kind of impossible, money spent and paid back have to balance, at least at some point.
If you look on-line at the list of transactions and payments, does it appear to be correct. Normally the only things missing from the running balance are payments that haven't posted yet, or purchases that have been logged but not cleared, so they don't add to the negative balance.
It would be a little bit of a pain but you can always put the payments and purchases in a spreadsheet and compare them to what you see on-line.
Another alternative would be to let that credit card rest for a month, only use one of the others. If you go through a month without making any purchases on that card, at the end of the next billing cycle it has to display a correct balance (or it's time to have a conversation with the credit card company).
I normally try to keep a small positive balance, there is no way I can miss a payment and get charged a penalty or interest that way. I make frequent payments throughout the month as I make purchases, which provides me with the security of a credit card but I treat it like a debit card. Things change though, one of my credit card companies will return any positive balance after a certain number of billing cycles (I get a check for a couple of dollars) so I have to keep an eye on that to keep the slightly positive balance. I haven't paid interest or a penalty since 1985 and that one was planned (new fridge, had to carry a balance for two months).
