Unfortunately this is an all too common occurence when you co-sign anything at all.  Really you should have notice to all that will impact you legally.  I would read the agreement that you signed (I hope that you have a copy) and if you do not, then ask the collections people to send you one.  Ask them to "validate the debt" in that you wish a copy of the original agreement, a copy of all the activity on the account and a copy of their agreement with the creditor allowing them to collect on their behalf.  Once you have toe documentation read it carefully and then you can figure out how to go from there.  Good luck.