letter from current debt collector

Backstory: We got sued for debt, was offered a settlement, to pay it off in payments, agreed, signed the paper work. We have been making payments, on-time, every month, online, on the debt collectors own website directly, and have all the receipts for every payment. Today, in informed delivery, there is letter from them, marked personal and confidential. What could they possibly want now? I am hoping it's just statement of payments made so far? Do they even do that? Collector is Johnson Mark.