Handed over despite making payments every month.

Last year I missed one payment on my Revolving credit account. I am retired and finances are very difficult. I called and arranged to make it up with FNB and I have been making the monthly payments regularly, which include an extra amount towards the shortfall. After being harassed for a while with Robocall messages, despite speaking to the representatives, I have now been handed over to attorneys who are demanding the full amount from me. And the amount they are demanding is 10K more than what my statement shows. After one missed payment, that I was paying off anyway??