I would agree with Anna as such sounds plausible (not too familiar with credit reporting issues) but I just wanted to point out that such would apply only if the account is being paid as required by either you or mom.
If the account is not being paid the lender may go after mom since she is also responsible and, presumably, has not filed bk.
Edt: Since you are in a 13, payment would only be allowed
by you if you provided for payment of this co-signed consumer debt through the Plan purusant to 11 USC 1301. Did you? If not and mom is not paying then, once the creditor gets relief from the co-debtor stay the creditor will go after mom for payment. You need to verify that the Plan pays the claim IN FULL.
Des.