Changes to interchange also means processor increases. On your August or September statement you probably had a notice stating that interchange rates (the true cost to process credit cards from the card issuing banks) were going to be updated in October (your October statement received in November). Generally the changes are minimal increases or changes to how certain cards settle at a designated card type. However, credit card processors use the opportunity to add fees and make increases while trying to mask them as interchange. Processors have even gone as far as to add the word "Interchange" to their surcharges to try to make it seem like a pass-through cost instead of an additional cost they are using to increase profits.
Pay close attention to your October statements to see if there any new fees show up or increases to your processor's rates. Mastercard and Visa post their interchange rates on their websites in a downloadable pdf for merchants, so any actual interchange updates are verifiable. merchantadvocate.com