Whoa, Nelly - hold on there. While I certainly wouldn't put it past the Rat or your ex to be pulling something shady, there MIGHT be a more benign explanation to all this. It wouldn't be the first time on these boards that OW's info somehow got mixed up with the LBSs; it might be something as simple as the ex having taken out a new credit card and put her on it as an additional user, or the Rat having put herself as "mrs. rat" on an apartment application.. Then the credit company ASSumes she's you.
If there was a more nefarious purpose, wouldn't something show up on your report - at least a stray card you didn't request???
Contact the credit reporting agencies and find out what you need to do to clear this up.
