How do I explain what the letter is?

I realized my student loans were overdue so I signed up for the new US repayment plan. They sent me something while I was away and my fiancé said on the phone “I thought this letter from US Department of Education was for me but it’s actually for you” I had given them my new address so I made sure I got the letter.

Me having overdue loans is going to cause big issues so I’d rather just start paying them and not say anything.

What do I say the letter was? I can’t remember if he thinks my loans are paid off.