Citibank Credit Card Complaint

Citibank Credit card department,

Closing/Cancelling account California

I closed my credit card account with Citi in -/-/2016. They even issued a refund check for the membership fee. To my surprise, i got a call from collection agency today letting me know that i am past due on the account. I login to my citi account and see that i have interest fee charge, late fee for 2 cycle. How is it possible when i closed the account? The interest charged was dated -/-/-. I dont even have my credit card number anymore because i already shredded the card -/-/. I closed the account because the unsatisfactory customer service i received from Citi. Mind you i had Citi prestige card for several years and paid $450.00 annual fee.